Mere Christianity by C.S. Lewis is our book. So far, we've read through Book 3, part 6: Christian Marriage.
He starts by pointing out that "the Christian doctrines on this subject [marriage] are extremely unpopular." Tell us about it! Lewis composed this line during Word War II. One wonders if he could even imagine the attack the Christian doctrines on marriage would come under seventy years later! Lewis continues:
"The Christian idea of marriage is based on Christ's words that a man and wife are to be regarded as a single organism--for that is what the words 'one flesh' would be in modern English. And the Christians believe that when He said this He was not expressing a sentiment but stating a fact--just as one is stating a fact when one says that a lock and its key are one mechanism, or that a violin and a bow are one musical instrument. The inventor of the human machine was telling us that its two halves, the male and the female, were made to be combined together in pairs, not simply on the sexual level, but totally combined. The monstrosity of sexual intercourse outside marriage is that those who indulge in it are trying to isolate one kind of union (the sexual) from all the other kinds of union which were intended to go along with it and make up the total union. The Christian attitude does not mean that there is anything wrong about sexual pleasure, any more than about the pleasure of eating. It means that you must not isolate that pleasure and try to get it by itself, any more than you ought to try to get the pleasures of taste without swallowing and digesting, by chewing things and spitting them out again." (pp 104-05)