Aquinas replies:
I answer that As stated above (153, 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.
