What it is
Lust is an inordinate craving for the pleasures of the body.
Why you do it
Your punishment in Hell will be
You’ll be smothered in fire and brimstone. Not kisses.
In the Summa Theologiae, medieval theologian Thomas Aquinas said of Lust:
…wherever there occurs a special kind of deformity whereby the venereal act is rendered unbecoming, there is a determinate species of lust. This may occur in two ways: First, through being contrary to right reason, and this is common to all lustful vices; secondly, because, in addition, it is contrary to the natural order of the venereal act as becoming to the human race: and this is called “the unnatural vice.”
I’m going to interrupt Thomas here and just say he had opinions on masturbation and gay marriage that you’d expect of a dude who was born is 1225. Or a fundamentalist born twenty years ago. Progress!