Summa Th. II-II EN Qu.153


We must next consider the vice of lust which is opposed to chastity: (1) Lust in general; (2) its species. Under the first head there are five points of inquiry:

(1) What is the matter of lust?

(2) Whether all copulation is unlawful?

(3) Whether lust is a mortal sin?

(4) Whether lust is a capital vice?

(5) Concerning its daughters.

Whether the matter of lust is only venereal desires and pleasures?

Objection: 1. It would seem that the matter of lust is not only venereal desires and pleasures. For Augustine says (Confess. ii, 6) that "lust affects to be called surfeit and abundance." But surfeit regards meat and drink, while abundance refers to riches. Therefore lust is not properly about venereal desires and pleasures.
2. Further, it is written (Pr 20,1): "Wine is a lustful [Douay: 'luxurious'] thing." Now wine is connected with pleasure of meat and drink. Therefore these would seem to be the matter of lust.
3. Further, lust is defined "as the desire of wanton pleasure" [*Alexander of Hales, Summ. Theol. ii, cxvli]. But wanton pleasure regards not only venereal matters but also many others. Therefore lust is not only about venereal desires and pleasures.

On the contrary To the lustful it is said (De Vera Relig. iii [*Written by St. Augustine]): "He that soweth in the flesh, of the flesh shall reap corruption." Now the sowing of the flesh refers to venereal pleasures. Therefore these belong to lust.
I answer that As Isidore says (Etym. x), "a lustful man is one who is debauched with pleasures." Now venereal pleasures above all debauch a man's mind. Therefore lust is especially concerned with such like pleasures.

Reply to Objection: 1. Even as temperance chiefly and properly applies to pleasures of touch, yet consequently and by a kind of likeness is referred to other matters, so too, lust applies chiefly to venereal pleasures, which more than anything else work the greatest havoc in a man's mind, yet secondarily it applies to any other matters pertaining to excess. Hence a gloss on Ga 5,19 says "lust is any kind of surfeit."
2. Wine is said to be a lustful thing, either in the sense in which surfeit in any matter is ascribed to lust, or because the use of too much wine affords an incentive to venereal pleasure.
3. Although wanton pleasure applies to other matters, the name of lust has a special application to venereal pleasures, to which also wantonness is specially applicable, as Augustine remarks (De Civ. xiv, 15,16).

Whether no venereal act can be without sin?

Objection: 1. It would seem that no venereal act can be without sin. For nothing but sin would seem to hinder virtue. Now every venereal act is a great hindrance to virtue. For Augustine says (Soliloq. i, 10): "I consider that nothing so casts down the manly mind from its height as the fondling of a woman, and those bodily contacts." Therefore, seemingly, no venereal act is without sin.
2. Further, any excess that makes one forsake the good of reason is sinful, because virtue is corrupted by "excess" and "deficiency" as stated in Ethic. ii, 2. Now in every venereal act there is excess of pleasure, since it so absorbs the mind, that "it is incompatible with the act of understanding," as the Philosopher observes (Ethic. vii, 11); and as Jerome [*Origen, Hom. vi in Num.; Cf. Jerome, Ep. cxxiii ad Ageruch.] states, rendered the hearts of the prophets, for the moment, insensible to the spirit of prophecy. Therefore no venereal act can be without sin.
3. Further, the cause is more powerful than its effect. Now original sin is transmitted to children by concupiscence, without which no venereal act is possible, as Augustine declares (De Nup. et Concup. i, 24). Therefore no venereal act can be without sin.

On the contrary Augustine says (De Bono Conjug. xxv): "This is a sufficient answer to heretics, if only they will understand that no sin is committed in that which is against neither nature, nor morals, nor a commandment": and he refers to the act of sexual intercourse between the patriarchs of old and their several wives. Therefore not every venereal act is a sin.
I answer that A sin, in human acts, is that which is against the order of reason. Now the order of reason consists in its ordering everything to its end in a fitting manner. Wherefore it is no sin if one, by the dictate of reason, makes use of certain things in a fitting manner and order for the end to which they are adapted, provided this end be something truly good. Now just as the preservation of the bodily nature of one individual is a true good, so, too, is the preservation of the nature of the human species a very great good. And just as the use of food is directed to the preservation of life in the individual, so is the use of venereal acts directed to the preservation of the whole human race. Hence Augustine says (De Bono Conjug. xvi): "What food is to a man's well being, such is sexual intercourse to the welfare of the whole human race." Wherefore just as the use of food can be without sin, if it be taken in due manner and order, as required for the welfare of the body, so also the use of venereal acts can be without sin, provided they be performed in due manner and order, in keeping with the end of human procreation.

Reply to Objection: 1. A thing may be a hindrance to virtue in two ways. First, as regards the ordinary degree of virtue, and as to this nothing but sin is an obstacle to virtue. Secondly, as regards the perfect degree of virtue, and as to this virtue may be hindered by that which is not a sin, but a lesser good. In this way sexual intercourse casts down the mind not from virtue, but from the height, i.e. the perfection of virtue. Hence Augustine says (De Bono Conjug. viii): "Just as that was good which Martha did when busy about serving holy men, yet better still that which Mary did in hearing the word of God: so, too, we praise the good of Susanna's conjugal chastity, yet we prefer the good of the widow Anna, and much more that of the Virgin Mary."
2. As stated above (Question [152], Article [2], ad 2; I-II 64,2), the mean of virtue depends not on quantity but on conformity with right reason: and consequently the exceeding pleasure attaching to a venereal act directed according to reason, is not opposed to the mean of virtue. Moreover, virtue is not concerned with the amount of pleasure experienced by the external sense, as this depends on the disposition of the body; what matters is how much the interior appetite is affected by that pleasure. Nor does it follow that the act in question is contrary to virtue, from the fact that the free act of reason in considering spiritual things is incompatible with the aforesaid pleasure. For it is not contrary to virtue, if the act of reason be sometimes interrupted for something that is done in accordance with reason, else it would be against virtue for a person to set himself to sleep. That venereal concupiscence and pleasure are not subject to the command and moderation of reason, is due to the punishment of the first sin, inasmuch as the reason, for rebelling against God, deserved that its body should rebel against it, as Augustine says (De Civ. Dei xiii, 13).
3. As Augustine says (De Civ. Dei xiii, 13), "the child, shackled with original sin, is born of fleshly concupiscence (which is not imputed as sin to the regenerate) as of a daughter of sin." Hence it does not follow that the act in question is a sin, but that it contains something penal resulting from the first sin.

Whether the lust that is about venereal acts can be a sin?

Objection: 1. It would seem that lust about venereal acts cannot be a sin. For the venereal act consists in the emission of semen which is the surplus from food, according to the Philosopher (De Gener. Anim. i, 18). But there is no sin attaching to the emission of other superfluities. Therefore neither can there be any sin in venereal acts.
2. Further, everyone can lawfully make what use he pleases of what is his. But in the venereal act a man uses only what is his own, except perhaps in adultery or rape. Therefore there can be no sin in venereal acts, and consequently lust is no sin.
3. Further, every sin has an opposite vice. But, seemingly, no vice is opposed to lust. Therefore lust is not a sin.

On the contrary The cause is more powerful than its effect. Now wine is forbidden on account of lust, according to the saying of the Apostle (Ep 5,18), "Be not drunk with wine wherein is lust [Douay: 'luxury']." Therefore lust is forbidden.Further, it is numbered among the works of the flesh: Ga 5,19 [Douay: 'luxury'].
I answer that The more necessary a thing is, the more it behooves one to observe the order of reason in its regard; wherefore the more sinful it becomes if the order of reason be forsaken. Now the use of venereal acts, as stated in the foregoing Article, is most necessary for the common good, namely the preservation of the human race. Wherefore there is the greatest necessity for observing the order of reason in this matter: so that if anything be done in this connection against the dictate of reason's ordering, it will be a sin. Now lust consists essentially in exceeding the order and mode of reason in the matter of venereal acts. Wherefore without any doubt lust is a sin.

Reply to Objection: 1. As the Philosopher says in the same book (De Gener. Anim. i, 18), "the semen is a surplus that is needed." For it is said to be superfluous, because it is the residue from the action of the nutritive power, yet it is needed for the work of the generative power. But the other superfluities of the human body are such as not to be needed, so that it matters not how they are emitted, provided one observe the decencies of social life. It is different with the emission of semen, which should be accomplished in a manner befitting the end for which it is needed.
2. As the Apostle says (1Co 6,20) in speaking against lust, "You are bought with a great price: glorify and bear God in your body." Wherefore by inordinately using the body through lust a man wrongs God Who is the Supreme Lord of our body. Hence Augustine says (De Decem. Chord. 10 [*Serm. ix (xcvi de Temp.)]): "God Who thus governs His servants for their good, not for His, made this order and commandment, lest unlawful pleasures should destroy His temple which thou hast begun to be."
3. The opposite of lust is not found in many, since men are more inclined to pleasure. Yet the contrary vice is comprised under insensibility, and occurs in one who has such a dislike for sexual intercourse as not to pay the marriage debt.

Whether lust is a capital vice?

Objection: 1. It seems that lust is not a capital vice. For lust is apparently the same as "uncleanness," according to a gloss on Ep 5,3 (2Co 12,21). But uncleanness is a daughter of gluttony, according to Gregory (Moral. xxxi, 45). Therefore lust is not a capital vice.
2. Further, Isidore says (De Summo Bono ii, 39) that "as pride of mind leads to the depravity of lust, so does humility of mind safeguard the chastity of the flesh." Now it is seemingly contrary to the nature of a capital vice to arise from another vice. Therefore lust is not a capital vice.
3. Further, lust is caused by despair, according to Ep 4,19, "Who despairing, have given themselves up to lasciviousness." But despair is not a capital vice; indeed, it is accounted a daughter of sloth, as stated above (Question [35], Article [4], ad 2). Much less, therefore, is lust a capital vice.

On the contrary Gregory (Moral. xxxi, 45) places lust among the capital vices.
I answer that As stated above (Question [148], Article [5]; I-II 84,3 I-II 84,4), a capital vice is one that has a very desirable end, so that through desire for that end, a man proceeds to commit many sins, all of which are said to arise from that vice as from a principal vice. Now the end of lust is venereal pleasure, which is very great. Wherefore this pleasure is very desirable as regards the sensitive appetite, both on account of the intensity of the pleasure, and because such like concupiscence is connatural to man. Therefore it is evident that lust is a capital vice.

Reply to Objection: 1. As stated above (Question [148], Article [6]), according to some, the uncleanness which is reckoned a daughter of gluttony is a certain uncleanness of the body, and thus the objection is not to the point. If, however, it denote the uncleanness of lust, we must reply that it is caused by gluttony materially---in so far as gluttony provides the bodily matter of lust---and not under the aspect of final cause, in which respect chiefly the capital vices are said to be the cause of others.
2. As stated above (Question [132], Article [4], ad 1), when we were treating of vainglory, pride is accounted the common mother of all sins, so that even the capital vices originate therefrom.
3. Certain persons refrain from lustful pleasures chiefly through hope of the glory to come, which hope is removed by despair, so that the latter is a cause of lust, as removing an obstacle thereto, not as its direct cause; whereas this is seemingly necessary for a capital vice.

Whether the daughters of lust are fittingly described?

Objection: 1. It would seem that the daughters of lust are unfittingly reckoned to be "blindness of mind, thoughtlessness, inconstancy, rashness, self-love, hatred of God, love of this world and abhorrence or despair of a future world." For mental blindness, thoughtlessness and rashness pertain to imprudence, which is to be found in every sin, even as prudence is in every virtue. Therefore they should not be reckoned especially as daughters of lust.
2. Further, constancy is reckoned a part of fortitude, as stated above (Question [128], ad 6; Question [137], Article [3]). But lust is contrary, not to fortitude but to temperance. Therefore inconstancy is not a daughter of lust.
3. Further, "Self-love extending to the contempt of God" is the origin of every sin, as Augustine says (De Civ. Dei xiv, 28). Therefore it should not be accounted a daughter of lust.
4. Further, Isidore [*Questions. in Deut., qu. xvi] mentions four, namely, "obscene," "scurrilous," "wanton" and "foolish talking." There the aforesaid enumeration would seem to be superfluous.

On the contrary stands the authority of Gregory (Moral. xxxi, 45).
I answer that When the lower powers are strongly moved towards their objects, the result is that the higher powers are hindered and disordered in their acts. Now the effect of the vice of lust is that the lower appetite, namely the concupiscible, is most vehemently intent on its object, to wit, the object of pleasure, on account of the vehemence of the pleasure. Consequently the higher powers, namely the reason and the will, are most grievously disordered by lust.Now the reason has four acts in matters of action. First there is simple understanding, which apprehends some end as good, and this act is hindered by lust, according to Da 13,56, "Beauty hath deceived thee, and lust hath perverted thy heart." In this respect we have "blindness of mind." The second act is counsel about what is to be done for the sake of the end: and this is also hindered by the concupiscence of lust. Hence Terence says (Eunuch., act 1, sc. 1), speaking of lecherous love: "This thing admits of neither counsel nor moderation, thou canst not control it by counseling." In this respect there is "rashness," which denotes absence of counsel, as stated above (Question [53], Article [3]). The third act is judgment about the things to be done, and this again is hindered by lust. For it is said of the lustful old men (Da 13,9): "They perverted their own mind . . . that they might not . . . remember just judgments." In this respect there is "thoughtlessness." The fourth act is the reason's command about the thing to be done, and this also is impeded by lust, in so far as through being carried away by concupiscence, a man is hindered from doing what his reason ordered to be done. [To this "inconstancy" must be referred.] [*The sentence in brackets is omitted in the Leonine edition.] Hence Terence says (Eunuch., act 1, sc. 1) of a man who declared that he would leave his mistress: "One little false tear will undo those words."On the part of the will there results a twofold inordinate act. One is the desire for the end, to which we refer "self-love," which regards the pleasure which a man desires inordinately, while on the other hand there is "hatred of God," by reason of His forbidding the desired pleasure. The other act is the desire for the things directed to the end. With regard to this there is "love of this world," whose pleasures a man desires to enjoy, while on the other hand there is "despair of a future world," because through being held back by carnal pleasures he cares not to obtain spiritual pleasures, since they are distasteful to him.

Reply to Objection: 1. According to the Philosopher (Ethic. vi, 5), intemperance is the chief corruptive of prudence: wherefore the vices opposed to prudence arise chiefly from lust, which is the principal species of intemperance.
2. The constancy which is a part of fortitude regards hardships and objects of fear; but constancy in refraining from pleasures pertains to continence which is a part of temperance, as stated above (Question [143]). Hence the inconstancy which is opposed thereto is to be reckoned a daughter of lust. Nevertheless even the first named inconstancy arises from lust, inasmuch as the latter enfeebles a man's heart and renders it effeminate, according to Os 4,11, "Fornication and wine and drunkenness take away the heart [Douay: 'understanding']." Vegetius, too, says (De Re Milit. iii) that "the less a man knows of the pleasures of life, the less he fears death." Nor is there any need, as we have repeatedly stated, for the daughters of a capital vice to agree with it in matter (cf. Question [35], Article [4], ad 2; Question [118], Article [8], ad 1; Question [148], Article [6]).
3. Self-love in respect of any goods that a man desires for himself is the common origin of all sins; but in the special point of desiring carnal pleasures for oneself, it is reckoned a daughter of lust.
4. The sins mentioned by Isidore are inordinate external acts, pertaining in the main to speech; wherein there is a fourfold inordinateness. First, on account of the matter, and to this we refer "obscene words": for since "out of the abundance of the heart the mouth speaketh" (Mt 12,34), the lustful man, whose heart is full of lewd concupiscences, readily breaks out into lewd words. Secondly, on account of the cause: for, since lust causes thoughtlessness and rashness, the result is that it makes a man speak without weighing or giving a thought to his words. which are described as "scurrilous." Thirdly, on account of the end: for since the lustful man seeks pleasure, he directs his speech thereto, and so gives utterance to "wanton words." Fourthly, on account of the sentiments expressed by his words, for through causing blindness of mind, lust perverts a man's sentiments, and so he gives way "to foolish talking," for instance, by expressing a preference for the pleasures he desires to anything else.


We must now consider the parts of lust, under which head there are twelve points of inquiry:

(1) Into what parts is lust divided?

(2) Whether simple fornication is a mortal sin?

(3) Whether it is the greatest of sins?

(4) Whether there is mortal sin in touches, kisses and such like seduction?

(5) Whether nocturnal pollution is a mortal sin?

(6) Of seduction;

(7) Of rape;

(8) Of adultery;

(9) Of incest;

(10) Of sacrilege;

(11) Of the sin against nature;

(12) Of the order of gravity in the aforesaid sins.

Whether six species are fittingly assigned to lust?

Objection: 1. It would seem that six species are unfittingly assigned to lust, namely, "simple fornication, adultery, incest, seduction, rape, and the unnatural vice." For diversity of matter does not diversify the species. Now the aforesaid division is made with regard to diversity of matter, according as the woman with whom a man has intercourse is married or a virgin, or of some other condition. Therefore it seems that the species of lust are diversified in this way.
2. Further, seemingly the species of one vice are not differentiated by things that belong to another vice. Now adultery does not differ from simple fornication, save in the point of a man having intercourse with one who is another's, so that he commits an injustice. Therefore it seems that adultery should not be reckoned a species of lust.
3. Further, just as a man may happen to have intercourse with a woman who is bound to another man by marriage, so may it happen that a man has intercourse with a woman who is bound to God by vow. Therefore sacrilege should be reckoned a species of lust, even as adultery is.
4. Further, a married man sins not only if he be with another woman, but also if he use his own wife inordinately. But the latter sin is comprised under lust. Therefore it should be reckoned among the species thereof.
5. Further, the Apostle says (2Co 12,21): "Lest again, when I come, God humble me among you, and I mourn many of them /that sinned before, and have not done penance for the uncleanness and fornication and lasciviousness that they have committed." Therefore it seems that also uncleanness and lasciviousness should be reckoned species of lust, as well as fornication.
6. Further, the thing divided is not to be reckoned among its parts. But lust is reckoned together with the aforesaid: for it is written (Ga 5,19): "The works of the flesh are manifest, which are fornication, uncleanness, immodesty, lust [Douay: 'luxury']." Therefore it seems that fornication is unfittingly reckoned a species of lust.

On the contrary The aforesaid division is given in the Decretals 36, qu. i [*Append. Grat. ad can. Lex illa].
I answer that I answer that As stated above (Question [153], Article [3]), the sin of lust consists in seeking venereal pleasure not in accordance with right reason. This may happen in two ways. First, in respect of the matter wherein this pleasure is sought; secondly, when, whereas there is due matter, other due circumstances are not observed. And since a circumstance, as such, does not specify a moral act, whose species is derived from its object which is also its matter, it follows that the species of lust must be assigned with respect to its matter or object.Now this same matter may be discordant with right reason in two ways. First, because it is inconsistent with the end of the venereal act. In this way, as hindering the begetting of children, there is the "vice against nature," which attaches to every venereal act from which generation cannot follow; and, as hindering the due upbringing and advancement of the child when born, there is "simple fornication," which is the union of an unmarried man with an unmarried woman. Secondly, the matter wherein the venereal act is consummated may be discordant with right reason in relation to other persons; and this in two ways. First, with regard to the woman, with whom a man has connection, by reason of due honor not being paid to her; and thus there is "incest," which consists in the misuse of a woman who is related by consanguinity or affinity. Secondly, with regard to the person under whose authority the woman is placed: and if she be under the authority of a husband, it is "adultery," if under the authority of her father, it is "seduction," in the absence of violence, and "rape" if violence be employed.These species are differentiated on the part of the woman rather than of the man, because in the venereal act the woman is passive and is by way of matter, whereas the man is by way of agent; and it has been stated above (Objection [1]) that the aforesaid species are assigned with regard to a difference of matter.

Reply to Objection: 1. The aforesaid diversity of matter is connected with a formal difference of object, which difference results from different modes of opposition to right reason, as stated above.
2. As stated above (I-II 18,7), nothing hinders the deformities of different vices concurring in the one act, and in this way adultery is comprised under lust and injustice. Nor is this deformity of injustice altogether accidental to lust: since the lust that obeys concupiscence so far as to lead to injustice, is thereby shown to be more grievous.
3. Since a woman, by vowing continence, contracts a spiritual marriage with God, the sacrilege that is committed in the violation of such a woman is a spiritual adultery. In like manner, the other kinds of sacrilege pertaining to lustful matter are reduced to other species of lust.
4. The sin of a husband with his wife is not connected with undue matter, but with other circumstances, which do not constitute the species of a moral act, as stated above (I-II 18,2).
5. As a gloss says on this passage, "uncleanness" stands for lust against nature, while "lasciviousness" is a man's abuse of boys, wherefore it would appear to pertain to seduction. We may also reply that "lasciviousness" relates to certain acts circumstantial to the venereal act, for instance kisses, touches, and so forth.
6. According to a gloss on this passage "lust" there signifies any kind of excess.

Whether simple fornication is a mortal sin?

Objection: 1. It would seem that simple fornication is not a mortal sin. For things that come under the same head would seem to be on a par with one another. Now fornication comes under the same head as things that are not mortal sins: for it is written (Ac 15,29): "That you abstain from things sacrificed to idols, and from blood, and from things strangled, and from fornication." But there is not mortal sin in these observances, according to 1Tm 4,4, "Nothing is rejected that is received with thanksgiving." Therefore fornication is not a mortal sin.
2. Further, no mortal sin is the matter of a Divine precept. But the Lord commanded (Os 1,2): "Go take thee a wife of fornications, and have of her children of fornications." Therefore fornication is not a mortal sin.
3. Further, no mortal sin is mentioned in Holy Writ without disapprobation. Yet simple fornication is mentioned without disapprobation by Holy Writ in connection with the patriarchs. Thus we read (Gn 16,4) that Abraham went in to his handmaid Agar; and further on (Gn 30,5-9) that Jacob went in to Bala and Zelpha the handmaids of his wives; and again (Gn 38,18) that Juda was with Thamar whom he thought to be a harlot. Therefore simple fornication is not a mortal sin.
4. Further, every mortal sin is contrary to charity. But simple fornication is not contrary to charity, neither as regards the love of God, since it is not a sin directly against. God, nor as regards the love of our neighbor, since thereby no one is injured. Therefore simple fornication is not a mortal sin.
5. Further, every mortal sin leads to eternal perdition. But simple fornication has not this result: because a gloss of Ambrose [*The quotation is from the Gloss of Peter Lombard, who refers it to St. Ambrose: whereas it is from Hilary the deacon] on 1Tm 4,8, "Godliness is profitable to all things," says: "The whole of Christian teaching is summed up in mercy and godliness: if a man conforms to this, even though he gives way to the inconstancy of the flesh, doubtless he will be punished, but he will not perish." Therefore simple fornication is not a mortal sin.
6. Further, Augustine says (De Bono Conjug. xvi) that "what food is to the well-being of the body, such is sexual intercourse to the welfare of the human race." But inordinate use of food is not always a mortal sin. Therefore neither is all inordinate sexual intercourse; and this would seem to apply especially to simple fornication, which is the least grievous of the aforesaid species.

On the contrary It is written (Tb 4,13): "Take heed to keep thyself . . . from all fornication, and beside thy wife never endure to know a crime." Now crime denotes a mortal sin. Therefore fornication and all intercourse with other than one's wife is a mortal sin.Further, nothing but mortal sin debars a man from God's kingdom. But fornication debars him, as shown by the words of the Apostle (Ga 5,21), who after mentioning fornication and certain other vices, adds: "They who do such things shall not obtain the kingdom of God." Therefore simple fornication is a mortal sin.Further, it is written in the Decretals (XXII, qu. i, can. Praedicandum): "They should know that the same penance is to be enjoined for perjury as for adultery, fornication, and wilful murder and other criminal offenses." Therefore simple fornication is a criminal or mortal sin.
I answer that Without any doubt we must hold simple fornication to be a mortal sin, notwithstanding that a gloss [*St. Augustine, Questions. in Deut., qu. 37] on Dt 23,17, says: "This is a prohibition against going with whores, whose vileness is venial." For instead of "venial" it should be "venal," since such is the wanton's trade. In order to make this evident, we must take note that every sin committed directly against human life is a mortal sin. Now simple fornication implies an inordinateness that tends to injure the life of the offspring to be born of this union. For we find in all animals where the upbringing of the offspring needs care of both male and female, that these come together not indeterminately, but the male with a certain female, whether one or several; such is the case with all birds: while, on the other hand, among those animals, where the female alone suffices for the offspring's upbringing, the union is indeterminate, as in the case of dogs and like animals. Now it is evident that the upbringing of a human child requires not only the mother's care for his nourishment, but much more the care of his father as guide and guardian, and under whom he progresses in goods both internal and external. Hence human nature rebels against an indeterminate union of the sexes and demands that a man should be united to a determinate woman and should abide with her a long time or even for a whole lifetime. Hence it is that in the human race the male has a natural solicitude for the certainty of offspring, because on him devolves the upbringing of the child: and this certainly would cease if the union of sexes were indeterminate.This union with a certain definite woman is called matrimony; which for the above reason is said to belong to the natural law. Since, however, the union of the sexes is directed to the common good of the whole human race, and common goods depend on the law for their determination, as stated above (I-II 90,2), it follows that this union of man and woman, which is called matrimony, is determined by some law. What this determination is for us will be stated in the Third Part of this work (XP, Question [50], seqq.), where we shall treat of the sacrament of matrimony. Wherefore, since fornication is an indeterminate union of the sexes, as something incompatible with matrimony, it is opposed to the good of the child's upbringing, and consequently it is a mortal sin.Nor does it matter if a man having knowledge of a woman by fornication, make sufficient provision for the upbringing of the child: because a matter that comes under the determination of the law is judged according to what happens in general, and not according to what may happen in a particular case.

Reply to Objection: 1. Fornication is reckoned in conjunction with these things, not as being on a par with them in sinfulness, but because the matters mentioned there were equally liable to cause dispute between Jews and Gentiles, and thus prevent them from agreeing unanimously. For among the Gentiles, fornication was not deemed unlawful, on account of the corruption of natural reason: whereas the Jews, taught by the Divine law, considered it to be unlawful. The other things mentioned were loathsome to the Jews through custom introduced by the law into their daily life. Hence the Apostles forbade these things to the Gentiles, not as though they were unlawful in themselves, but because they were loathsome to the Jews, as stated above (I-II 103,4, ad 3).
2. Fornication is said to be a sin, because it is contrary to right reason. Now man's reason is right, in so far as it is ruled by the Divine Will, the first and supreme rule. Wherefore that which a man does by God's will and in obedience to His command, is not contrary to right reason, though it may seem contrary to the general order of reason: even so, that which is done miraculously by the Divine power is not contrary to nature, though it be contrary to the usual course of nature. Therefore just as Abraham did not sin in being willing to slay his innocent son, because he obeyed God, although considered in itself it was contrary to right human reason in general, so, too, Osee sinned not in committing fornication by God's command. Nor should such a copulation be strictly called fornication, though it be so called in reference to the general course of things. Hence Augustine says (Confess. iii, 8): "When God commands a thing to be done against the customs or agreement of any people, though it were never done by them heretofore, it is to be done"; and afterwards he adds: "For as among the powers of human society, the greater authority is obeyed in preference to the lesser, so must God in preference to all."
3. Abraham and Jacob went in to their handmaidens with no purpose of fornication, as we shall show further on when we treat of matrimony (XP, Question [65], Article [5], ad 2). As to Juda there is no need to excuse him, for he also caused Joseph to be sold.
4. Simple fornication is contrary to the love of our neighbor, because it is opposed to the good of the child to be born, as we have shown, since it is an act of generation accomplished in a manner disadvantageous to the future child.
5. A person, who, while given to works of piety, yields to the inconstancy of the flesh, is freed from eternal loss, in so far as these works dispose him to receive the grace to repent, and because by such works he makes satisfaction for his past inconstancy; but not so as to be freed by pious works, if he persist in carnal inconstancy impenitent until death.
6. One copulation may result in the begetting of a man, wherefore inordinate copulation, which hinders the good of the future child, is a mortal sin as to the very genus of the act, and not only as to the inordinateness of concupiscence. On the other hand, one meal does not hinder the good of a man's whole life, wherefore the act of gluttony is not a mortal sin by reason of its genus. It would, however, be a mortal sin, if a man were knowingly to partake of a food which would alter the whole condition of his life, as was the case with Adam.Nor is it true that fornication is the least of the sins comprised under lust, for the marriage act that is done out of sensuous pleasure is a lesser sin.

Summa Th. II-II EN Qu.153