Chastity is the most unpopular of the Christian virtues. There is no getting away from it: the old 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 that 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 the stage. Now suppose you came 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?
One critic said that if he found a country in which such striptease acts with food were popular, he would conclude that the people of that country were starving. He meant, of course, to imply that such things as the strip-tease act resulted not from sexual corruption but from sexual starvation. I agree with him that if, in some strange land, we found that similar acts with mutton chops were popular, one of the possible explanations which would occur to me would be famine. But the next step would be to test our hypothesis by finding out whether, in fact, much or little food was being consumed in that country. If the evidence showed that a good deal was being eaten, then of course we should have to abandon the hypothesis of starvation and try to think of another one. In the same way, before accepting sexual starvation as the cause of the strip-tease, we should have to look for evidence that there is in fact more sexual abstinence in our age than in those ages when things like the strip-tease were unknown. But surely there is no such evidence. Contraceptives have made sexual indulgence far less costly within marriage and far safer outside it than ever before, and public opinion is less hostile to illicit unions and even to perversion than it has been since Pagan times. Nor is the hypothesis of "starvation" the only one we can imagine. Everyone knows that the sexual appetite, like our other appetites, grows by indulgence. Starving men may think much about food, but so do gluttons; the gorged, as well as the famished, like titillations.
"Sexual Morality" (chapter 5 of Christian Behavior)