Objection 1. It would seem that incest is not a determinate species of lust. For incest ['Incestus' is equivalent to 'in-castus = 'unchaste'] takes its name from being a privation of chastity. But all kinds of lust are opposed to chastity. Therefore it seems that incest is not a species of lust, but is lust itself in general.
Objection 2. Further, it is stated in the Decretals (XXXVI, qu. 1 [Cf. Append. Grat. ad can. Lex illa]) that "incest is intercourse between a man and a woman related by consanguinity or affinity." Now affinity differs from consanguinity. Therefore it is not one but several species of lust.
Objection 3. Further, that which does not, of itself, imply a deformity, does not constitute a determinate species of vice. But intercourse between those who are related by consanguinity or affinity does not, of itself, contain any deformity, else it would never have been lawful. Therefore incest is not a determinate species of lust.
On the contrary, The species of lust are distinguished according to the various conditions of women with whom a man has unlawful intercourse. Now incest implies a special condition on the part of the woman, because it is unlawful intercourse with a woman related by consanguinity or affinity as stated (Objection 2). Therefore incest is a determinate species of lust.
I answer that, As stated above (A1,6) wherever we find something incompatible with the right use of venereal actions, there must needs be a determinate species of lust. Now sexual intercourse with women related by consanguinity or affinity is unbecoming to venereal union on three counts. First, because man naturally owes a certain respect to his parents and therefore to his other blood relations, who are descended in near degree from the same parents: so much so indeed that among the ancients, as Valerius Maximus relates [Dict. Fact. Memor. ii, 1, it was not deemed right for a son to bathe with his father, lest they should see one another naked. Now from what has been said (142, 4; 151, 4), it is evident that in venereal acts there is a certain shamefulness inconsistent with respect, wherefore men are ashamed of them. Wherefore it is unseemly that such persons should be united in venereal intercourse. This reason seems to be indicated (Leviticus 18:7) where we read: "She is thy mother, thou shalt not uncover her nakedness," and the same is expressed further on with regard to others.
The second reason is because blood relations must needs live in close touch with one another. Wherefore if they were not debarred from venereal union, opportunities of venereal intercourse would be very frequent and thus men's minds would be enervated by lust. Hence in the Old Law [Leviticus 18] the prohibition was apparently directed specially to those persons who must needs live together.
The third reason is, because this would hinder a man from having many friends: since through a man taking a stranger to wife, all his wife's relations are united to him by a special kind of friendship, as though they were of the same blood as himself. Wherefore Augustine says (De Civ. Dei xv, 16): "The demands of charity are most perfectly satisfied by men uniting together in the bonds that the various ties of friendship require, so that they may live together in a useful and becoming amity; nor should one man have many relationships in one, but each should have one."
Aristotle adds another reason (2 Polit. ii): for since it is natural that a man should have a liking for a woman of his kindred, if to this be added the love that has its origin in venereal intercourse, his love would be too ardent and would become a very great incentive to lust: and this is contrary to chastity. Hence it is evident that incest is a determinate species of lust.
Reply to Objection 1. Unlawful intercourse between persons related to one another would be most prejudicial to chastity, both on account of the opportunities it affords, and because of the excessive ardor of love, as stated in the Article. Wherefore the unlawful intercourse between such persons is called "incest" antonomastically.
Reply to Objection 2. Persons are related by affinity through one who is related by consanguinity: and therefore since the one depends on the other, consanguinity and affinity entail the same kind of unbecomingness.
Reply to Objection 3. There is something essentially unbecoming and contrary to natural reason in sexual intercourse between persons related by blood, for instance between parents and children who are directly and immediately related to one another, since children naturally owe their parents honor. Hence the Philosopher instances a horse (De Animal. ix, 47) which covered its own mother by mistake and threw itself over a precipice as though horrified at what it had done, because some animals even have a natural respect for those that have begotten them. There is not the same essential unbecomingness attaching to other persons who are related to one another not directly but through their parents: and, as to this, becomingness or unbecomingness varies according to custom, and human or Divine law: because, as stated above (Article 2), sexual intercourse, being directed to the common good, is subject to law. Wherefore, as Augustine says (De Civ. Dei xv, 16), whereas the union of brothers and sisters goes back to olden times, it became all the more worthy of condemnation when religion forbade it.
Source: Summa Theologica (New Advent)