Justice, as I said before, includes the keeping of promises. Now everyone who has been married in a church has made a public, solemn promise to stick to his (or her) partner till death. The duty of keeping that promise has no special connection with sexual morality: it is in the same position as any other promise. If, as modern people are always telling us, the sexual impulse is just like all our other impulses, then it ought to be treated like all our other impulses; and as their indulgence is controlled by our promises, so should its be. If, as I think, it is not like all our other impulses, but is morbidly inflamed, then we should be especially careful not to let it lead us into dishonesty.
—C.S. Lewis, Mere Christianity