Chapter III: On Consummated Sins of Lust Against Nature

91. "As often as the order itself of nature is violated in the matter of lust, a sin against nature is present. But this can happen in two ways: first, because the use of the proper matter is impeded; and this happens in every voluntary emission of semen apart from a due union of male and female, which is called uncleanness, which some call softness. Secondly, because the conjunction of male and female is not observed; and this either as to the due sex, and thus a man sins with a man, or a woman with a woman, as the Apostle says to the Romans, chapter 1, and this is called sodomy; or as to due species, as when a man has commerce with a beast, and this is called bestiality." St. Thomas, 2. 2. q. 154. a. 11.

The species, therefore, of this sin against nature are: pollution, bestiality, sodomy. Onanism, however, of which we spoke above in n. 69., is true and properly so-called pollution, containing a special malice, as it were, of adultery; and hence it is even more grievous than solitary pollution. Nay more, it is also opposed to the end of marriage, and to the right of the other spouse.

These sins, according to the more common judgment of theologians, are of themselves more grievous than sins according to nature, because they are more opposed to the order of nature. Yet, by reason of circumstances, a sin according to nature can be more grievous; thus incest, sacrilege, adultery, by reason of the added malice, can be more grievous than simple pollution. "Softness, sodomy, and bestiality are sins of the same ultimate species, as to that which is common to them, namely, that they are against nature; but they are distinct according to species, according to diverse ways of being against nature." St. Thomas, 2. 2. q. 154. a. 11. "The sin against nature is the gravest among the species of lust." St. Thomas, ibid. a. 12.