I think a secularist has only one substitute left for God, only one experience in a desacrilized world that still gives him something like the mystical, self-transcending thrill of ecstasy that God designed all souls to have forever, and to long for until they have it. Unless he is a surfer, that experience has to be sex. We’re designed for more than happiness; we’re designed for joy. Aquinas writes, with simple logic, “Man cannot live without joy. That is why one deprived of true spiritual joys must spill over to carnal pleasures.”
Drugs and alcohol are attractive because they claim to feed the same need. The lack the ontological greatness of sex, but they provide the same semi-mystical thrill: the transcendence of reason and self-consciousness. I do not mean this merely as moral condemnation, but as psychological analysis.
In fact, though they sound shocking, I think the addict is closer to the deepest truth than the mere moralist. He is looking for the very best thing in some of the very worst places. His demand for a state in which he transcends morality is very wrong, but it’s also very right. For we are designed for something beyond morality, something in which morality will be transformed. Mystical union with God. Sex is a sign and appetizer of that.
Moral absolutists must never forget that morality, though absolute, is not ultimate. It is not our Summum Bonum. Sinai is not the Promised Land; Jerusalem is. And in the New Jerusalem, what finally happens as the last chapter of human history is a wedding between the Lamb and His bride. Deprived of this Jerusalem, we must buy into Babylon. If we do not worship God, we will worship idols, for we are by nature worshippers.
Again, this is by Peter Kreeft, from here.