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."
, Augustine says (De adult. conjug. [The quotation is from Cap. Adulterii xxxii, qu. 7. Cf. Augustine, De Bono Conjugali, viii.]) that "of all these," namely the sins belonging to lust, "that which is against nature is the worst."