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.
Objection 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.
Objection 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.
Objection 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.
Objection 5. Further, the Apostle says (2 Corinthians 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.
Objection 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 (Galatians 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 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. On 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.
Reply to Objection 2. As stated above (I-II, 18, 07), 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.
Reply to Objection 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. On like manner, the other kinds of sacrilege pertaining to lustful matter are reduced to other species of lust.
Reply to Objection 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).
Reply to Objection 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.
Reply to Objection 6. According to a gloss on this passage "lust" there signifies any kind of excess.
Source: Summa Theologica (New Advent)