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 (Obj. 1) that the aforesaid species are assigned with regard to a difference of matter.
Reply Obj. 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 Obj. 2: As stated above (I-II, Q. 18, A. 7), 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 Obj. 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. In like manner, the other kinds of sacrilege pertaining to lustful matter are reduced to other species of lust.
Reply Obj. 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, Q. 18, A. 2).
Reply Obj. 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 Obj. 6: According to a gloss on this passage "lust" there signifies any kind of excess. _______________________
SECOND ARTICLE [II-II, Q. 154, Art. 2]
Whether Simple Fornication Is a Mortal Sin?
Objection 1: It would seem that simple fornication is not a mortal sin. For things that come under the same head would seem to be on a par with one another. Now fornication comes under the same head as things that are not mortal sins: for it is written (Acts 15:29): "That you abstain from things sacrificed to idols, and from blood, and from things strangled, and from fornication." But there is not mortal sin in these observances, according to 1 Tim. 4:4, "Nothing is rejected that is received with thanksgiving." Therefore fornication is not a mortal sin.