Is Desire For Sin Itself Sin? A Response

I received a very thoughtful and charitable bit of feedback from someone wondering whether the desire for sin is itself sin. Below is my response, edited to preserve the anonymity of my interlocutor. 

One can experience the attractiveness of a forbidden object because, on one level of analysis, the object is good, and therefore desire it insofar as it is good on that level of analysis, but reject it because on another level of analysis, it is not good *for me.* I would not think that such a person had sinned. For example, pleasure is good, and I might desire the pleasure I believe I would have were I to have sex with my neighbor's husband, but if my will does not assent to the act or even to the experiencing of the desire, I do not think I have sinned. The desire shows a deformation of soul--it is an inordinate desire--but not a sin. Similarly, we might say that when Jesus was hungry in the wilderness he desired bread insofar as it is in itself a good and would conduce to his flourishing from one perspective, but that he did not assent to the desiring for bread, since in the final analysis fasting was his good.

It might be objected that "if homosexuality is not sinful, then are bestiality and pedophilia not sinful?" I do not think that my argument is at all susceptible to a reductio ad absurdum of this kind. I do not hold the object of desire in the case of desire for same-sex sexual activity to be neutral, nor in the case of desire for sexual activity with a child or a beast, nor, for that matter, in the case of desire for opposite-sex sexual activity with someone other than one’s own spouse. Each case shows a derangement of the affections, but I do not believe such derangements in themselves, apart from the operation of the will to assent to them or act upon them, to be sins. I agree that these derangements or disorders are not all created equally. Perhaps—but I am only speculating here—that the order of derangement is related to the degree to which reality would have to be bent for the desire to be a virtuous one. Perhaps the desire for sexual activity with one’s betrothed is almost—but not quite—innocent. Desire for sexual activity with a person of the same sex is more deviant in that one or the other would have to be a different sex for the act to be licit—a much greater change (presumably) than the taking of a vow. (Although now that I write the words, my belief in marriage as a sacrament makes me want to take them back.) Bestiality is yet more deviant in that, per impossibile, a change of nature of one or either party would be required for the act to be licit. Pedophilia is the most deviant of the bunch, in that the act violates the will of an innocent; it entails an act of rape in a way that the others do not (bestiality may, for all I know, involve the violation of a will, but a beast has not the capacity to be harmed that a child has). 

To get back to the main point, I think the game is given away when one is forced to admit that the desire for sex outside of heterosexual marriage may be a disordered result of the fall but is not a desire that affects the will. I take it nearly as a truism—it seems to me to be on such solid ground biblically and traditionally—that sins are functions of the will. Is it not so? The desire for same-sex sexual activity is a “sinful desire” in that it inclines the will toward a sin, but it is not itself a sin. I just re-read Thomas’s section on virtue and vice in the Summa, and he writes in Prima Secundae Partis, question 71, Article 3 that a man is justly punished for a vicious act but not for a vicious habit. How much more, then, is man not punished for a vicious orientation, which (unlike a habit) he received (at least at first) passively. Since all sin is justly punished, a vicious habit or orientation, though conducive to sin, is not itself sin.