Objection 1. It would seem that the unnatural vice is not a species of lust. For no mention of the vice against nature is made in the enumeration given above (1, Objection 1). Therefore it is not a species of lust.
Objection 2. Further, lust is contrary to virtue; and so it is comprised under vice. But the unnatural vice is comprised not under vice, but under bestiality, according to the Philosopher (Ethic. vii, 5). Therefore the unnatural vice is not a species of lust.
Objection 3. Further, lust regards acts directed to human generation, as stated above (Question 153, Article 2): Whereas the unnatural vice concerns acts from which generation cannot follow. Therefore the unnatural vice is not a species of lust.
On the contrary, It is reckoned together with the other species of lust (2 Corinthians 12:21) where we read: "And have not done penance for the uncleanness, and fornication, and lasciviousness," where a gloss says: "Lasciviousness, i.e., unnatural lust."
I answer that, As stated above (A6,9) wherever there occurs a special kind of deformity whereby the venereal act is rendered unbecoming, there is a determinate species of lust. This may occur in two ways: First, through being contrary to right reason, and this is common to all lustful vices; secondly, because, in addition, it is contrary to the natural order of the venereal act as becoming to the human race: and this is called "the unnatural vice." This may happen in several ways. First, by procuring pollution, without any copulation, for the sake of venereal pleasure: this pertains to the sin of "uncleanness" which some call "effeminacy." Secondly, by copulation with a thing of undue species, and this is called "bestiality." Thirdly, by copulation with an undue sex, male with male, or female with female, as the Apostle states (Romans 1:27): and this is called the "vice of sodomy." Fourthly, by not observing the natural manner of copulation, either as to undue means, or as to other monstrous and bestial manners of copulation.
Reply to Objection 1. There we enumerated the species of lust that are not contrary to human nature: wherefore the unnatural vice was omitted.
Reply to Objection 2. Bestiality differs from vice, for the latter is opposed to human virtue by a certain excess in the same matter as the virtue, and therefore is reducible to the same genus.
Reply to Objection 3. The lustful man intends not human generation but venereal pleasures. It is possible to have this without those acts from which human generation follows: and it is that which is sought in the unnatural vice.
Source: Summa Theologica (New Advent)