Almost everything Edward Feser posts is worth linking to, but I think this topic is especially in need of clear explanation in today’s world. It’s a wonderful analysis and explanation of why sexual sins are “such a big deal”:
[One of the] morally significant aspect[s] of sex, … that the unique intensity of sexual pleasure can lead us to act irrationally, is perhaps less often discussed these days. So let’s talk about that.
* * *
[P]recisely because sexual pleasure is unusually intense, it is even more likely than other pleasures are to impair our ability to perceive what is true and good when what we take pleasure in is something that is in fact bad. In particular, habitually indulging one’s desire to carry out sexual acts that are disordered will tend to make it harder and harder for one to see that they are disordered. For one thing, the pleasure a person repeatedly takes in those acts will give the acts the false appearance of goodness; for another, the person will be inclined to look for reasons to regard the acts as good or at least harmless, and disinclined to look for, or give a dispassionate hearing to, reasons to think them bad. Hence indulgence in disordered sexual behavior has a tendency to impair one’s ability to perceive the true and the good, particularly in matters of sexual morality. In short, sexual vice makes you stupid.
* * *
[I]in contemporary intellectual life most people know nothing of, or at best know only crude caricatures of, [the general metaphysical framework underlying traditional natural law theory]…. Hence they fail to understand the rational foundations of traditional sexual morality.
But the Thomist is bound to judge that mere intellectual error is not the only problem. For it’s not just that people in contemporary Western society commonly disagree, at an intellectual level, with the natural law theorist’s judgments about what is disordered. It’s that they commonly act in ways that natural law theory says are disordered. And if such behavior has a tendency to impair one’s capacity to perceive what is true and good, especially where sex is concerned, then it follows that widespread rejection of traditional sexual morality is bound to have as much to do with the sort of cognitive corruption that Aquinas calls “blindness of mind” as it does with the making of honest intellectual mistakes. That people who don’t behave in accordance with traditional sexual moral norms also don’t believe that these norms have any solid intellectual foundation is thus in no way surprising. On the contrary, that’s exactly what natural law theory itself predicts will happen.
* * *
And thus it is no surprise that Christian theologians have traditionally emphasized the dangers sexual sins pose to one’s immortal soul. This is not because such sins are the worst sins — they are not — but rather because the pleasure associated with them makes them very easy to fall into and, if they become habitual, very difficult to get out of. (Churchmen who want to downplay the significance of sexual sins in the name of compassion are thus acting in a way that is in fact anything but compassionate.)
I highly recommend reading the whole thing, as these excerpts come nowhere near doing it justice.
Edward Feser, “What’s the deal with sex? Part II“, Edward Feser blog, February 6, 2015.