Summa - Supplement 1083

Whether consanguinity is an impediment to marriage by virtue of the natural law?


Objection 1: It would seem that consanguinity is not by natural law an impediment to marriage. For no woman can be more akin to a man than Eve was to Adam, since of her did he say (
Gn 2,23): "This now is bone of my bones and flesh of my flesh." Yet Eve was joined in marriage to Adam. Therefore as regards the natural law no consanguinity is an impediment to marriage.

Objection 2: Further, the natural law is the same for all. Now among the uncivilized nations no person is debarred from marriage by reason of consanguinity. Therefore, as regards the law of nature, consanguinity is no impediment to marriage.

Objection 3: Further, the natural law is what "nature has taught all animals," as stated at the beginning of the Digests (i, ff. De just. et jure). Now brute animals copulate even with their mother. Therefore it is not of natural law that certain persons are debarred from marriage on account of consanguinity.

Objection 4: Further, nothing that is not contrary to one of the goods of matrimony is an impediment to marriage. But consanguinity is not contrary to any of the goods of marriage. Therefore it is not an impediment thereto.

Objection 5: Further, things which are more akin and more similar to one another are better and more firmly united together. Now matrimony is a kind of union. Since then consanguinity is a kind of kinship, it does not hinder marriage but rather strengthens the union.

On the contrary, According to the natural law whatever is an obstacle to the good of the offspring is an impediment to marriage. Now consanguinity hinders the good of the offspring, because in the words of Gregory (Regist., epis. xxxi) quoted in the text (Sent. iv, D, 40): "We have learnt by experience that the children of such a union cannot thrive." Therefore according to the law of nature consanguinity is an impediment to matrimony.

Further, that which belongs to human nature when it was first created is of natural law. Now it belonged to human nature from when it was first created that one should be debarred from marrying one's father or mother: in proof of which it was said (Gn 2,24): "Wherefore a man shall leave father and mother": which cannot be understood of cohabitation, and consequently must refer to the union of marriage. Therefore consanguinity is an impediment to marriage according to the natural law.

I answer that, In relation to marriage a thing is said to be contrary to the natural law if it prevents marriage from reaching the end for which it was instituted. Now the essential and primary end of marriage is the good of the offspring. and this is hindered by a certain consanguinity, namely that which is between father and daughter, or son and mother. It is not that the good of the offspring is utterly destroyed, since a daughter can have a child of her father's semen and with the father rear and teach that child in which things the good of the offspring consists, but that it is not effected in a becoming way. For it is out of order that a daughter be mated to her father in marriage for the purpose of begetting and rearing children, since in all things she ought to be subject to her father as proceeding from him. Hence by natural law a father and mother are debarred from marrying their children; and the mother still more than the father, since it is more derogatory to the reverence due to parents if the son marry his mother than if the father marry his daughter; since the wife should be to a certain extent subject to her husband. The secondary essential end of marriage is the curbing of concupiscence; and this end would be forfeit if a man could marry any blood-relation, since a wide scope would be afforded to concupiscence if those who have to live together in the same house were not forbidden to be mated in the flesh. Wherefore the Divine law debars from marriage not only father and mother, but also other kinsfolk who have to live in close intimacy with one another and ought to safeguard one another's modesty. The Divine law assigns this reason (Lv 18,10): "Thou shalt not uncover the nakedness" of such and such a one, "because it is thy own nakedness."

But the accidental end of marriage is the binding together of mankind and the extension of friendship: for a husband regards his wife's kindred as his own. Hence it would be prejudicial to this extension of friendship if a man could take a woman of his kindred to wife since no new friendship would accrue to anyone from such a marriage. Wherefore, according to human law and the ordinances of the Church, several degrees of consanguinity are debarred from marriage.

Accordingly it is clear from what has been said that consanguinity is by natural law an impediment to marriage in regard to certain persons, by Divine law in respect of some, and by human law in respect of others.

Reply to Objection 1: Although Eve was formed from Adam she was not Adam's daughter, because she was not formed from him after the manner in which it is natural for a man to beget his like in species, but by the Divine operation, since from Adam's rib a horse might have been formed in the same way as Eve was. Hence the natural connection between Eve and Adam was not so great as between daughter and father, nor was Adam the natural principle of Eve as a father is of his daughter.

Reply to Objection 2: That certain barbarians are united carnally to their parents does not come from the natural law but from the passion of concupiscence which has clouded the natural law in them.

Reply to Objection 3: Union of male and female is said to be of natural law, because nature has taught this to animals: yet she has taught this union to various animals in various ways according to their various conditions. But carnal copulation with parents is derogatory to the reverence due to them. For just as nature has instilled into parents solicitude in providing for their offspring, so has it instilled into the offspring reverence towards their parents: yet to no kind of animal save man has she instilled a lasting solicitude for his children or reverence for parents; but to other animals more or less, according as the offspring is more or less necessary to its parents, or the parents to their offspring. Hence as the Philosopher attests (De Animal. ix, 47) concerning the camel and the horse, among certain animals the son abhors copulation with its mother as long as he retains knowledge of her and a certain reverence for her. And since all honest customs of animals are united together in man naturally, and more perfectly than in other animals, it follows that man naturally abhors carnal knowledge not only of his mother, but also of his daughter, which is, however, less against nature, as stated above.

Moreover consanguinity does not result from carnal procreation in other animals as in man, as stated above (Article [1], ad 5). Hence the comparison fails.

Reply to Objection 4: It has been shown how consanguinity between married persons is contrary to the goods of marriage. Hence the Objection proceeds from false premises.

Reply to Objection 5: It is not unreasonable for one of two unions to be hindered by the other, even as where there is identity there is not likeness. In like manner the tie of consanguinity may hinder the union of marriage.

Whether the degrees of consanguinity that are an impediment to marriage could be fixed by the Church?


Objection 1: It would seem that the degrees of consanguinity that are an impediment to marriage could not be fixed by the Church so as to reach to the fourth degree. For it is written (
Mt 19,6): "What God hath joined together let no man put asunder." But God joined those together who are married within the fourth degree of consanguinity, since their union is not forbidden by the Divine law. Therefore they should not be put asunder by a human law.

Objection 2: Further, matrimony is a sacrament as also is baptism. Now no ordinance of the Church could prevent one who is baptized from receiving the baptismal character, if he be capable of receiving it according to the Divine law. Therefore neither can an ordinance of the Church forbid marriage between those who are not forbidden to marry by the Divine law.

Objection 3: Further, positive law can neither void nor extend those things which are natural. Now consanguinity is a natural tie which is in itself of a nature to impede marriage. Therefore the Church cannot by its ordinance permit or forbid certain people to marry, any more than she can make them to be kin or not kin.

Objection 4: Further, an ordinance of positive law should have some reasonable cause, since it is for this reasonable cause that it proceeds from the natural law. But the causes that are assigned for the number of degrees seem altogether unreasonable, since they bear no relation to their effect; for instance, that consanguinity be an impediment as far as the fourth degree on account of the four elements as far as the sixth degree on account of the six ages of the world, as far as the seventh degree on account of the seven days of which all time is comprised. Therefore seemingly this prohibition is of no force.

Objection 5: Further, where the cause is the same there should be the same effect. Now the causes for which consanguinity is an impediment to marriage are the good of the offspring, the curbing of concupiscence, and the extension of friendship, as stated above (Article [3]), which are equally necessary for all time. Therefore the degrees of consanguinity should have equally impeded marriage at all times: yet this is not true since consanguinity is now an impediment to marriage as far as the fourth degree, whereas formerly it was an impediment as far as the seventh.

Objection 6: Further, one and the same union cannot be a kind of sacrament and a kind of incest. But this would be the case if the Church had the power of fixing a different number in the degrees which are an impediment to marriage. Thus if certain parties related in the fifth degree were married when that degree was an impediment, their union would be incestuous, and yet this same union would be a marriage afterwards when the Church withdrew her prohibition. And the reverse might happen if certain degrees which were not an impediment were subsequently to be forbidden by the Church. Therefore seemingly the power of the Church does not extend to this.

Objection 7: Further, human law should copy the Divine law. Now according to the Divine law which is contained in the Old Law, the prohibition of degrees does not apply equally in the ascending and descending lines: since in the Old Law a man was forbidden to marry his father's sister but not his brother's daughter. Therefore neither should there remain now a prohibition in respect of nephews and uncles.

On the contrary, Our Lord said to His disciples (Lc 10,16): "He that heareth you heareth Me." Therefore a commandment of the Church has the same force as a commandment of God. Now the Church sometimes has forbidden and sometimes allowed certain degrees which the Old Law did not forbid. Therefore those degrees are an impediment to marriage.

Further, even as of old the marriages of pagans were controlled by the civil law, so now is marriage controlled by the laws of the Church. Now formerly the civil law decided which degrees of consanguinity impede marriage, and which do not. Therefore this can be done now by a commandment of the Church.

I answer that, The degrees within which consanguinity has been an impediment to marriage have varied according to various times. For at the beginning of the human race father and mother alone were debarred from marrying their children, because then mankind were few in number, and then it was necessary for the propagation of the human race to be ensured with very great care, and consequently only such persons were to be debarred as were unfitted for marriage even in respect of its principal end which is the good of the offspring, as stated above (Article [3]). Afterwards however, the human race having multiplied, more persons were excluded by the law of Moses, for they already began to curb concupiscence. Wherefore as Rabbi Moses says (Doc. Perp. iii, 49) all those persons were debarred from marrying one another who are wont to live together in one household, because if a lawful carnal intercourse were possible between them, this would prove a very great incentive to lust. Yet the Old Law permitted other degrees of consanguinity, in fact to a certain extent it commanded them; to wit that each man should take a wife from his kindred, in order to avoid confusion of inheritances: because at that time the Divine worship was handed down as the inheritance of the race. But afterwards more degrees were forbidden by the New Law which is the law of the spirit and of love, because the worship of God is no longer handed down and spread abroad by a carnal birth but by a spiritual grace: wherefore it was necessary that men should be yet more withdrawn from carnal things by devoting themselves to things spiritual, and that love should have a yet wider play. Hence in olden times marriage was forbidden even within the more remote degrees of consanguinity, in order that consanguinity and affinity might be the sources of a wider natural friendship; and this was reasonably extended to the seventh degree, both because beyond this it was difficult to have any recollection of the common stock, and because this was in keeping with the sevenfold grace of the Holy Ghost. Afterwards, however, towards these latter times the prohibition of the Church has been restricted to the fourth degree, because it became useless and dangerous to extend the prohibition to more remote degrees of consanguinity. Useless, because charity waxed cold in many hearts so that they had scarcely a greater bond of friendship with their more remote kindred than with strangers: and it was dangerous because through the prevalence of concupiscence and neglect men took no account of so numerous a kindred, and thus the prohibition of the more remote degrees became for many a snare leading to damnation. Moreover there is a certain fittingness in the restriction of the above prohibition to the fourth degree. First because men are wont to live until the fourth generation, so that consanguinity cannot lapse into oblivion, wherefore God threatened (Ex 20,5) to visit the parent's sins on their children to the third and fourth generation. Secondly, because in each generation the blood, the identity of which causes consanguinity, receives a further addition of new blood, and the more another blood is added the less there is of the old. And because there are four elements, each of which is the more easily mixed with another, according as it is more rarefied it follows that at the first admixture the identity of blood disappears as regards the first element which is most subtle; at the second admixture, as regards the second element; at the third, as to the third element; at the fourth, as to the fourth element. Thus after the fourth generation it is fitting for the carnal union to be repeated.

Reply to Objection 1: Even as God does not join together those who are joined together against the Divine command, so does He not join together those who are joined together against the commandment of the Church, which has the same binding force as a commandment of God.

Reply to Objection 2: Matrimony is not only a sacrament but also fulfills an office; wherefore it is more subject to the control of the Church's ministers than baptism which is a sacrament only: because just as human contracts and offices are controlled by human laws, so are spiritual contracts and offices controlled by the law of the Church.

Reply to Objection 3: Although the tie of consanguinity is natural, it is not natural that consanguinity forbid carnal intercourse, except as regards certain degrees, as stated above (Article [3]). Wherefore the Church's commandment does not cause certain people to be kin or not kin, because they remain equally kin at all times: but it makes carnal intercourse to be lawful or unlawful at different times for different degrees of consanguinity.

Reply to Objection 4: The reasons assigned are given as indicating aptness and congruousness rather than causality and necessity.

Reply to Objection 5: The reason for the impediment of consanguinity is not the same at different times: wherefore that which it was useful to allow at one time, it was beneficial to forbid at another.

Reply to Objection 6: A commandment does not affect the past but the future. Wherefore if the fifth degree which is now allowed were to be forbidden at any time, those in the fifth degree who are married would not have to separate, because no impediment supervening to marriage can annul it; and consequently a union which was a marriage from the first would not be made incestuous by a commandment of the Church. In like manner, if a degree which is now forbidden were to be allowed, such a union would not become a marriage on account of the Church's commandment by reason of the former contract, because they could separate if they wished. Nevertheless, they could contract anew, and this would be a new union.

Reply to Objection 7: In prohibiting the degrees of consanguinity the Church considers chiefly the point of view of affection. And since the reason for affection towards one's brother's son is not less but even greater than the reasons for affection towards one's father's brother, inasmuch as the son is more akin to the father than the father to the son (Ethic. viii, 12), therefore did the Church equally prohibit the degrees of consanguinity in uncles and nephews. On the other hand the Old Law in debarring certain persons looked chiefly to the danger of concupiscence arising from cohabitation; and debarred those persons who were in closer intimacy with one another on account of their living together. Now it is more usual for a niece to live with her uncle than an aunt with her nephew: because a daughter is more identified with her father, being part of him, whereas a sister is not in this way identified with her brother, for she is not part of him but is born of the same parent. Hence there was not the same reason for debarring a niece and an aunt.



We must consider next the impediment of affinity. Under this head there are eleven points of inquiry:

(1) Whether affinity results from matrimony?

(2) Whether it remains after the death of husband or wife?

(3) Whether it is caused through unlawful intercourse?

(4) Whether it arises from a betrothal?

(5) Whether affinity is caused through affinity?

(6) Whether affinity is an impediment to marriage?

(7) Whether affinity in itself admits of degrees?

(8) Whether its degrees extend as far as the degrees of consanguinity?

(9) Whether marriages of persons related to one another by consanguinity or affinity should always be dissolved by divorce?

(10) Whether the process for the dissolution of like marriages should always be by way of accusation?

(11) Whether witnesses should be called in such a case?

Whether a person contracts affinity through the marriage of a blood-relation?


Objection 1: It would seem that a person does not contract affinity through the marriage of a blood-relation. For "the cause of a thing being so is yet more so." Now the wife is not connected with her husband's kindred except by reason of the husband. Since then she does not contract affinity with her husband, neither does she contract it with her husband's kindred.

Objection 2: Further, if certain things be separate from one another and something be connected with one of them, it does not follow that it is connected with the other. Now a person's blood relations are separate from one another. Therefore it does not follow, if a certain woman be married to a certain man, that she is therefore connected with all his kindred.

Objection 3: Further, relations result from certain things being united together. Now the kindred of the husband do not become united together by the fact of his taking a wife. Therefore they do not acquire any relationship of affinity.

On the contrary, Husband and wife are made one flesh. Therefore if the husband is related in the flesh to all his kindred, for the same reason his wife will be related to them all.

Further, this is proved by the authorities quoted in the text (Sent. iv, D, 41).

I answer that, A certain natural friendship is founded on natural fellowship. Now natural fellowship, according to the Philosopher (Ethic. viii, 12), arises in two ways; first, from carnal procreation; secondly, from connection with orderly carnal procreation, wherefore he says (Ethic. viii, 12) that the friendship of a husband towards his wife is natural. Consequently even as a person through being connected with another by carnal procreation is bound to him by a tie of natural friendship, so does one person become connected with another through carnal intercourse. But there is a difference in this, that one who is connected with another through carnal procreation, as a son with his father, shares in the same common stock and blood, so that a son is connected with his father's kindred by the same kind of tie as the father was, the tie, namely of consanguinity, albeit in a different degree on account of his being more distant from the stock: whereas one who is connected with another through carnal intercourse does not share in the same stock, but is as it were an extraneous addition thereto: whence arises another kind of tie known by the name of "affinity." This is expressed in the verse:

Marriage makes a new kind of connection,

While birth makes a new degree, because, to wit, the person begotten is in the same kind of relationship, but in a different degree, whereas through carnal intercourse he enters into a new kind of relationship.

Reply to Objection 1: Although a cause is more potent than its effect, it does not always follow that the same name is applicable to the cause as to the effect, because sometimes that which is in the effect, is found in the cause not in the same but in a higher way; wherefore it is not applicable to both cause and effect under the same name or under the same aspect, as is the case with all equivocal effective causes. Thus, then, the union of husband and wife is stronger than the union of the wife with her husband's kindred, and yet it ought not to be named affinity, but matrimony which is a kind of unity; even as a man is identical with himself, but not with his kinsman.

Reply to Objection 2: Blood-relations are in a way separate, and in a way connected: and it happens in respect of their connection that a person who is connected with one of them is in some way connected with all of them. But on account of their separation and distance from one another it happens that a person who is connected with one of them in one way is connected with another in another way, either as to the kind of connection or as to the degree.

Reply to Objection 3: Further, a relation results sometimes from a movement in each extreme, for instance fatherhood and sonship, and a relation of this kind is really in both extremes. Sometimes it results from the movement of one only, and this happens in two ways. In one way when a relation results from the movement of one extreme without any movement previous or concomitant of the other extreme; as in the Creator and the creature, the sensible and the sense, knowledge and the knowable object: and then the relation is in one extreme really and in the other logically only. In another way when the relation results from the movement of one extreme without any concomitant movement, but not without a previous movement of the other; thus there results equality between two men by the increase of one, without the other either increasing or decreasing then, although previously he reached his actual quantity by some movement or change, so that this relation is founded really in both extremes. It is the same with consanguinity and affinity, because the relation of brotherhood which results in a grown child on the birth of a boy, is caused without any movement of the former's at the time, but by virtue of that previous movement of his wherein he was begotten; wherefore at the time it happens that there results in him the aforesaid relation through the movement of another. Likewise because this man descends through his own birth from the same stock as the husband, there results in him affinity with the latter's wife, without any new change in him.

Whether affinity remains after the death of husband or wife?


Objection 1: It would seem that affinity does not remain after the death of husband or wife, between the blood-relations of husband and wife or "vice versa." Because if the cause cease the effect ceases. Now the cause of affinity was the marriage, which ceases after the husband's death, since then "the woman . . . is loosed from the law of the husband" (
Rm 7,2). Therefore the aforesaid affinity ceases also.

Objection 2: Further, consanguinity is the cause of affinity. Now the consanguinity of the husband with his blood-relations ceases at his death. Therefore, the wife's affinity with them ceases also.

On the contrary, Affinity is caused by consanguinity. Now consanguinity binds persons together for all time as long as they live. Therefore affinity does so also: and consequently affinity (between two persons) is not dissolved through the dissolution of the marriage by the death of a third person.

I answer that, A relation ceases in two ways: in one way through the corruption of its subject, in another way by the removal of its cause; thus likeness ceases when one of the like subjects dies, or when the quality that caused the likeness is removed. Now there are certain relations which have for their cause an action, or a passion or movement (Metaph. v, 20): and some of these are caused by movement, through something being moved actually; such is the relation between mover and moved: some of them are caused through something being adapted to movement, for instance the relations between the motive power and the movable, or between master and servant; and some of them result from something, having been moved previously, such as the relation between father and son, for the relation between them is caused not by (the con) being begotten now, but by his having been begotten. Now aptitude for movement and for being moved is transitory; whereas the fact of having been moved is everlasting, since what has been never ceases having been. Consequently fatherhood and sonship are never dissolved through the removal of the cause, but only through the corruption of the subject, that is of one of the subjects. The same applies to affinity, for this is caused by certain persons having been joined together not by their being actually joined. Wherefore it is not done away, as long as the persons between whom affinity has been contracted survive, although the person die through whom it was contracted.

Reply to Objection 1: The marriage tie causes affinity not only by reason of actual union, but also by reason of the union having been effected in the past.

Reply to Objection 2: Consanguinity is not the chief cause of affinity, but union with a blood-relation, not only because that union is now, but because it has been. Hence the argument does not prove.

Whether unlawful intercourse causes affinity?


Objection 1: It would seem that unlawful intercourse does not cause affinity. For affinity is an honorable thing. Now honorable things do not result from that which is dishonorable. Therefore affinity cannot be caused by a dishonorable intercourse.

Objection 2: Further, where there is consanguinity there cannot be affinity; since affinity is a relationship between persons that results from carnal intercourse and is altogether void of blood-relationship. Now if unlawful intercourse were a cause of affinity, it would sometimes happen that a man would contract affinity with his blood-relations and with himself: for instance when a man is guilty of incest with a blood-relation. Therefore affinity is not caused by unlawful intercourse.

Objection 3: Further, unlawful intercourse is according to nature or against nature. Now affinity is not caused by unnatural unlawful intercourse as decided by law (can. Extraordinaria, xxxv, qu. 2,3). Therefore it is not caused only by unlawful intercourse according to nature.

On the contrary, He who is joined to a harlot is made one body (
1Co 6,16). Now this is the reason why marriage caused affinity. Therefore unlawful intercourse does so for the same reason.

Further, carnal intercourse is the cause of affinity, as shown by the definition of affinity, which definition is as follows: Affinity is the relationship of persons which results from carnal intercourse and is altogether void of blood-relationship. But there is carnal copulation even in unlawful intercourse. Therefore unlawful intercourse causes affinity.

I answer that, According to the Philosopher (Ethic. viii, 12) the union of husband and wife is said to be natural chiefly on account of the procreation of offspring, and secondly on account of the community of works: the former of which belongs to marriage by reason of carnal copulation, and the latter, in so far as marriage is a partnership directed to a common life. Now the former is to be found in every carnal union where there is a mingling of seeds, since such a union may be productive of offspring, but the latter may be wanting. Consequently since marriage caused affinity, in so far as it was a carnal mingling, it follows that also an unlawful intercourse causes affinity in so far as it has something of natural copulation.

Reply to Objection 1: In an unlawful intercourse there is something natural which is common to fornication and marriage, and in this respect it causes affinity. There is also something which is inordinate whereby it differs from marriage, and in this respect it does not cause affinity. Hence affinity remains honorable, although its cause is in a way dishonorable.

Reply to Objection 2: There is no reason why diverse relations should not be in the same subject by reason of different things. Consequently there can be affinity and consanguinity between two persons, not only on account of unlawful but also on account of lawful intercourse: for instance if a blood-relation of mine on my father's side marries a blood-relation of mine on my mother's side. Hence in the above definition the words "which is altogether void of blood-relationship" apply to affinity as such. Nor does it follow that a man by having intercourse with his blood-relation contracts affinity with himself, since affinity, like consanguinity, requires diversity of subjects, as likeness does.

Reply to Objection 3: In unnatural copulation there is no mingling of seeds that makes generation possible: wherefore a like intercourse does not cause affinity.

Whether affinity is caused by betrothal?


Objection 1: It would seem that affinity cannot be caused by betrothal. For affinity is a lasting tie: whereas a betrothal is sometimes broken off. Therefore it cannot cause affinity.

Objection 2: Further if the hymen be penetrated without the deed being consummated, affinity is not contracted. Yet this is much more akin to carnal intercourse than a betrothal. Therefore betrothal does not cause affinity.

Objection 3: Further, betrothal is nothing but a promise of future marriage. Now sometimes there is a promise of future marriage without affinity being contracted, for instance if it take place before the age of seven years; or if a man having a perpetual impediment of impotence promise a woman future marriage; or if a like promise be made between persons to whom marriage is rendered unlawful by a vow; or in any other way whatever. Therefore betrothal cannot cause affinity.

On the contrary, Pope Alexander (cap. Ad audiendem, De spons. et matrim.) forbade a certain woman to marry a certain man, because she had been betrothed to his brother. Now this would not be the case unless affinity were contracted by betrothal. Therefore, etc.

I answer that, Just as a betrothal has not the conditions of a perfect marriage, but is a preparation for marriage, so betrothal causes not affinity as marriage does, but something like affinity. This is called "the justice of public honesty," which is an impediment to marriage even as affinity and consanguinity are, and according to the same degrees, and is defined thus: "The justice of public honesty is a relationship arising out of betrothal, and derives its force from ecclesiastical institution by reason of its honesty." This indicates the reason of its name as well as its cause, namely that this relationship was instituted by the Church on account of its honesty.

Reply to Objection 1: Betrothal, by reason not of itself but of the end to which it is directed, causes this kind of affinity known as "the justice of public honesty": wherefore just as marriage is a lasting tie, so is the aforesaid kind of affinity.

Reply to Objection 2: In carnal intercourse man and woman become one flesh by the mingling of seeds. Wherefore it is not every invasion or penetration of the hymen that causes affinity to be contracted, but only such as is followed by a mingling of seeds. But marriage causes affinity not only on account of carnal intercourse, but also by reason of the conjugal fellowship, in respect of which also marriage is according to nature. Consequently affinity results from the marriage contract itself expressed in words of the present and before its consummation, and in like manner there results from betrothal, which is a promise of conjugal fellowship, something akin to affinity, namely the justice of public honesty.

Reply to Objection 3: All those impediments which void a betrothal prevent affinity being contracted through a promise of marriage. Hence whether he who actually promises marriage be lacking in age, or be under a solemn vow of continence or any like impediment, no affinity nor anything akin to it results because the betrothal is void. If however, a minor, laboring under insensibility or malefice, having a perpetual impediment, is betrothed before the age of puberty and after the age of seven years, with a woman who is of age, from such a contract there results the impediment called "justice of public honesty," because at the time the impediment was not actual, since at that age the boy who is insensible is equally impotent in respect of the act in question.

Summa - Supplement 1083