The foundations of traditional sexual morality, like the foundations of all morality, are to be found in classical natural law theory. I set out the basic lines of argument in my essay “In Defense of the Perverted Faculty Argument,” which appears in my book Neo-Scholastic Essays. The title notwithstanding, the perverted faculty argument is by no means the whole of the natural law understanding of sexual morality, but only a part. It is an important and unjustly maligned part of it, however, as I show in the essay. Along the way I criticize purported alternative approaches to defending traditional sexual morality, such as the so-called “New Natural Law Theory.” Anyway, you can now read the essay online. After you’ve done so, you might follow up with some other things I’ve written on the subject of sexual morality.