Of course there is more to this. His thought is not completed in just that brief passage - but it's something to think about until I add the rest of the story.
There are two reasons why I do not want to deal with Christian marriage. The first is that Christian doctrines on this subject are extremely unpopular. The second is that I have never been married myself, (he was married later in life after this book was written) and therefore, can only speak at it secondhand. But in spite of that, I feel I can hardly leave the subject out in an account of Christian morals.
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 a lock and a 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 tasting without swallowing and digesting, by chewing things and spitting them out again.