Words from one of the greatest Christian thinkers...

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 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 striptease 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 everyone 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?
They tell you sex has become a mess because it was hushed up. But for the last twenty years it has not been hushed up. It has been chattered about all day long. Yet it is still in a mess. If hushing up had been the cause of the trouble, ventilation would have set it right. But it has not. I think it is the other way round. I think the human race originally hushed it up because it had become such a mess. Modern people are always saying, ‘Sex in nothing to be ashamed of.’ They may mean two things. They may mean ‘There is nothing to be ashamed of in the fact that the human race reproduces itself in a certain way, nor in the fact that it gives pleasure.’ If they mean that, they are right. Christianity says the same. It is not the thing, nor the pleasure, that is the trouble. The old Christian teachers said that if man had never fallen, sexual pleasure, instead of being less than it is now, would actually have been greater. I know some muddle-headed Christians have talked as if Christianity thought that sex, or the body, or pleasure, were bad in themselves. But they were wrong. Christianity is almost the only one of the great religions which thoroughly approves of the body - which believes that matter is good, that God Himself once took on a human body, that some kind of body is going to be given to us even in Heaven and is going to be an essential part of our happiness, our beauty, and our energy. Christianity has glorified marriage more than any other religion: and nearly all the greatest love poetry in world has been produced by Christians. If anyone says that sex, in itself, is bad, Christianity contradicts him at once. But, of course, when people say, ‘Sex in nothing to be ashamed of’, they mean ‘the state into which the sexual instinct has not got is nothing to be ashamed of’…
I think it is everything to be ashamed of. There is nothing to be ashamed of in enjoying your food: there would be everything to be ashamed of if half the world made food the main interest of their lives…There are people who want to keep our sex instinct inflamed in order to make money out of us. Because, of course, a man with an obsession is a man who has very little sales resistance.

- C.S. Lewis, The Business of Heaven