Here is another short excerpt from the classic book by C. S. Lewis about what Christians believe, Mere Christianity. This passage has to deal with sexual morality:
Chastity is the most unpopular of the Christian virtues. There is no getting away from it; 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.