Why is lust a sin? Several thoughts come to mind. Mainly, it's a source of frustration, like greed or covetousness, that gets in your way, obsesses you, keeps you from moving forward. It wastes your time, and I think that's the reason a lot of people, as they get older and their time on earth gets shorter, are glad to see it go. I think a character in a Kurt Vonnegut Jr. book says that losing desire is like getting off a bucking bronco. And I'm reminded of what Luis Bunuel said when asked what he would say if God offered to restore his virility: "No thank you, but please repair my lungs and liver so I can continue to smoke and drink."

There are likely no two men on this earth who were ever more dissimilar than William Blake and the Marquis de Sade, one a great religious mystic much given to seeing angels, the other a florid pornographer who revelled in rape and mutilation and just general all-out disgust. Both had some kind of general belief that man is ruled by desire:

"Those who restrain desire," Blake wrote in The Marriage of Heaven and Hell, "do so because theirs is weak enough to be restrained; and the restrainer or reason usurps its place & governs the unwilling.

"And being restraind it by degrees becomes passive till it is only the shadow of desire."

De Sade probably lived by this decree, and spent many years in prison as a result; Blake probably didn't and lived to an old and, according to Harold Bloom, happy old age. You can, actually, restrain desire and prosper; the only thing you can't do, really, is flee lust. There's that old saw of Martin Luther's: "You can't stop the birds from flying over your head, but you can keep them from building a nest in your hair." Actually, I think the nest will be made; it's the baby birds you want to avoid. You can refuse to act on desire, but desire itself won't be chased from the yard. It's like Joe Mantegna said in House of Games: "The things we want, the things we feel. We can do themn or not do them, but we can't hide them."