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 (1 Corinthians 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.
Source: Summa Theologica (New Advent)