Objection 1. It would seem that affinity also is a cause of affinity. For Pope Julius I says (cap. Contradicimus 35, qu. iii): "No man may marry his wife's surviving blood-relation": and it is said in the next chapter (cap. Porro duorum) that "the wives of two cousins are forbidden to marry, one after the other, the same husband." But this is only on account of affinity being contracted through union with a person related by affinity. Therefore affinity is a cause of affinity.
Objection 2. Further, carnal intercourse makes persons akin even as carnal procreation, since the degrees of affinity and consanguinity are reckoned equally. But consanguinity causes affinity. Therefore affinity does also.
Objection 3. Further, things that are the same with one and the same are the same with one another. But the wife contracts the same relations with all her husband's kindred. Therefore all her husband's kindred are made one with all who are related by affinity to the wife, and thus affinity is the cause of affinity.
Objection 4.On the contrary, If affinity is caused by affinity a man who has connection with two women can marry neither of them, because then the one would be related to the other by affinity. But this is false. Therefore affinity does not cause affinity.
Objection 5. Further, if affinity arose out of affinity a man by marrying another man's widow would contract affinity with all her first husband's kindred, since she is related to them by affinity. But this cannot be the case because he would become especially related by affinity to her deceased husband. Therefore, etc.
Objection 6. Further, consanguinity is a stronger tie than affinity. But the blood-relations of the wife do not become blood-relations of the husband. Much less, therefore, does affinity to the wife cause affinity to her blood-relations, and thus the same conclusion follows.
I answer that, There are two ways in which one thing proceeds from another: in one way a thing proceeds from another in likeness of species, as a man is begotten of a man: in another way one thing proceeds from another, not in likeness of species; and this process is always towards a lower species, as instanced in all equivocal agents. The first kind of procession, however often it be repeated, the same species always remains: thus if one man be begotten of another by an act of the generative power, of this man also another man will be begotten, and so on. But the second kind of procession, just as in the first instance it produces another species, so it makes another species as often as it is repeated. Thus by movement from a point there proceeds a line and not a point, because a point by being moved makes a line; and from a line moved lineally, there proceeds not a line but a surface, and from a surface a body, and in this way the procession can go no further. Now in the procession of kinship we find two kinds whereby this tie is caused: one is by carnal procreation, and this always produces the same species of relationship; the other is by the marriage union, and this produces a different kind of relationship from the beginning: thus it is clear that a married woman is related to her husband's blood-relations not by blood but by affinity. Wherefore if this kind of process be repeated, the result will be not affinity but another kind of relationship; and consequently a married party contracts with the affines of the other party a relation not of affinity but of some other kind which is called affinity of the second kind. And again if a person through marriage contracts relationship with an affine of the second kind, it will not be affinity of the second kind, but of a third kind, as indicated in the verse quoted above (Article 1). Formerly these two kinds were included in the prohibition, under the head of the justice of public honesty rather than under the head of affinity, because they fall short of true affinity, in the same way as the relationship arising out of betrothal. Now however they have ceased to be included in the prohibition, which now refers only to the first kind of affinity in which true affinity consists.
Reply to Objection 1. A husband contracts affinity of the first kind with his wife's male blood-relation, and affinity of the second kind with the latter's wife: wherefore if the latter man dies the former cannot marry his widow on account of the second kind of affinity. Again if a man A marry a widow B, C, a relation of her former husband being connected with B by the first kind of affinity, contracts affinity of the second kind with her husband A; and D, the wife of this relation C being connected, by affinity of the second kind, with B, this man's wife contracts affinity of the third kind with her husband A. And since the third kind of affinity was included in the prohibition on account of a certain honesty more than by reason of affinity, the canon (cap. Porro duorum 35, qu. iii) says: "The justice of public honesty forbids the wives of two cousins to be married to the same man, the one after the other." But this prohibition is done away with.
Reply to Objection 2. Although carnal intercourse is a cause of people being connected with one another, it is not the same kind of connection.
Reply to Objection 3. The wife contracts the same connection with her husband's relatives as to the degree but not as to the kind of connection. Since however the arguments in the contrary sense would seem to show that no tie is caused by affinity, we must reply to them lest the time-honored prohibition of the Church seem unreasonable.
Reply to Objection 4. As stated above, a woman does not contract affinity of the first kind with the man to whom she is united in the flesh, wherefore she does not contract affinity of the second kind with a woman known by the same man; and consequently if a man marry one of these women, the other does not contract affinity of the third kind with him. And so the laws of bygone times did not forbid the same man to marry successively two women known by one man.
Reply to Objection 5. As a man is not connected with his wife by affinity of the first kind, so he does not contract affinity of the second kind with the second husband of the same wife. Wherefore the argument does not prove.
Reply to Objection 6. One person is not connected with me through another, except they be connected together. Hence through a woman who is affine to me, no person becomes connected with me, except such as is connected with her. Now this cannot be except through carnal procreation from her, or through connection with her by marriage: and according to the olden legislation, I contracted some kind of connection through her in both ways: because her son even by another husband becomes affine to me in the same kind and in a different degree of affinity, as appears from the rule given above: and again her second husband becomes affine to me in the second kind of affinity. But her other blood-relations are not connected with him, but she is connected with them, either as with father or mother, inasmuch as she descends from them, or, as with her brothers, as proceeding from the same principle; wherefore the brother or father of my affine does not become affine to me in any kind of affinity.
Source: Summa Theologica (New Advent)