"The Christian rule is, 'Either marriage, with complete faithfulness to your partner, or else total abstinence.' Now this is so difficult and so contrary to our instincts, that obviously either Christianity is wrong or our sexual instinct, as it now is, has gone wrong. One or the other. Of course, being a Christian, I think it is the instinct which has gone wrong.

But I have other Reasons for thinking so. The biological purpose of sex is children, just as the biological purpose of eating is to repair the body. Now if we eat whenever we feel inclined and just as much as we want, it is quite true most of us will eat too much: but not terrifically too much. One man may eat enough for two, but he does not eat enough for ten. The appetite goes a little beyond its biological purpose, but not enormously. But if a healthy young man indulged his sexual appetite whenever he felt inclined, and if each act produced a baby, then in ten years he might easily populate a small village. This appetite is in ludicrous and preposterous excess of its function.

Or take it another way. You can get a large audience together for a strip-tease act - that is, to watch a girl undress on stage. Now suppose you come to a country where you could fill a theatre by simply bringing a covered plate on to the stage and then slowly lifting the cover so as to let every one see, just before the lights went out, that it contained a mutton chop or a bit of bacon, would you not think that in that country something had gone wrong with the appetite for food? And would not anyone who had grown up in a different world think there was something equally queer about the state of the sex instinct among us?"

- From Mere Christianity, by C.S. Lewis