Lewis on Sexual Morality and Marriage

The Christian rule of chastity must not be confused with the social rule of "modesty" (in one sense of that word); i.e. propriety, or decency. The social rule of propriety lays down how much of the human body should be displayed and what sujects can be referred to, and in what words, according to the customs of a given social circle. Thus, while the rule of chastity is the same for all Christians at all times, the rule of propriety changes.
"Sexual Morality," Mere Christitanity, 1943, New York: Macmillan Publishing, 1952, p. 88.

Chastity is the most unpopular of Christians 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."
"Sexual Morality," Mere Christitanity, 1943, New York: Macmillan Publishing, 1952, p. 89.

But perversions of the sex instinct are numerous, hard to cure, and frightful. I am sorry to have to go into all these details, but I must. The reason why I must is that you and I, for the last twenty years, have been fed all day long on good solid lies about sex. We have been told till one is sick of hearing it, that sexual desire is in the same state as any of our other natural desires and that if only we abandon the silly old Victorian idea of hushing it up, everything in the garden will be lovely. It is not true. The moment you look at the facts, and away from the propaganda, you see that it is not.
"Sexual Morality," Mere Christitanity, 1943, New York: Macmillan Publishing, 1952, pp 90-91.

Christianity has glorified marriage more than any other religion: and nearly all the greatest love poetry in the world has been produced by Christians. If anyone says that sex, in itself, is bad, Christianity contradicts him at once.
"Sexual Morality," Mere Christitanity, 1943, New York: Macmillan Publishing, 1952, p. 91.

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.
"Christian Marriage," Mere Christitanity, 1943, New York: Macmillan Publishing, 1952, p. 95.

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 are intended to go along with it and make up the total union.
"Christian Marriage," Mere Christitanity, 1943, New York: Macmillan Publishing, 1952, p. 96.

The idea that "being in love" is the only reason for remaining married really leaves no room for marriage as a contract or promise at all. If love is the whole thing, then the promise can add nothing; and if it adds nothing, then it should not be made.
"Christian Marriage," Mere Christitanity, 1943, New York: Macmillan Publishing, 1952, p. 97.

And, of course, the promise, made when I am in love and because I am in love, to be true to the beloved as long as I live, commits one to being true even if I cease to be in love. A promise must be about things that I can do, about actions: no one can promise to go on feeling in a certain way.
"Christian Marriage," Mere Christitanity, 1943, New York: Macmillan Publishing, 1952, p. 98.

But, of course, ceasing to be "in love" need not mean ceasing to love. Love in this second sense--love as distinct from "being in love" is not merely a feeling. It is a deep unity, maintained by the will and deliberately strengthened by habit; reinforced by (in Christian marriages) the grace which both parents ask, and receive, from God. . . . "Being in love" first moved them to promise fidelity: this quieter love enables them to keep the promise.
"Christian Marriage," Mere Christitanity, 1943, New York: Macmillan Publishing, 1952, p. 99.

A great many people seem to think that if you are a Christian yourself you should try to make divorce difficult for every one. I do not think that. At least I know I should be very angry if the Mohammedans tried to prevent the rest of us from drinking wine. My own view is that the Churches should frankly recognise that the majority of the British people are not Christians and, therefore, cannot be expect to live Christian lives.
"Christian Marriage," Mere Christitanity, 1943, New York: Macmillan Publishing, 1952, pp. 101-102.

In Christian marriage the man is said to be the "head." Two questions obviously arise here. (1) Why should there be a head at all--why not equality? (2) Why should it be the man?
"Christian Marriage," Mere Christitanity, 1943, New York: Macmillan Publishing, 1952, p. 102.

'After all', said Clare, 'they had a right to happiness.'

We were discussing something that once happened in our own neighborhood. Mr A. had deserted Mrs A. and got his divorce in order to marry Mrs B., who had likewise got her divorce in order to marry Mr A. And there was certainly no doubt that Mr A. and Mrs B. were very much in love with one another. If they continued to be in love, and if nothing went wrong with their health or their income, they might reasonably expect to be very happy.
"We Have No 'Right to Happiness," God in the Dock: Essays on Theology and Ethics, ed. Walter Hooper, Grand Rapids, MI: William B. Eerdmans Publishing Company, 1970, p. 317

I don't think it is obvious that people have the unlimited 'right to happiness' . . .

For one thing, I believe that Clare, when she says 'happiness', means simply and solely 'sexual happiness.'
"We Have No 'Right to Happiness," God in the Dock: Essays on Theology and Ethics, ed. Walter Hooper, Grand Rapids, MI: William B. Eerdmans Publishing Company, 1970, p. 319

Our sexual impulses are thus being put in a position of preposterous privilege. The sexual motive is taken to condone all sorts of behaviour which, if it had any other end in view, would be condemned as merciless, treacherous and unjust.
"We Have No 'Right to Happiness," God in the Dock: Essays on Theology and Ethics, ed. Walter Hooper, Grand Rapids, MI: William B. Eerdmans Publishing Company, 1970, p. 320

Every experienced adult knows this [the feelings that passion will last forever are deceptive] to be so as regards all erotic passions . . . . We discount the world-without-end pretensions of our friends' amours easily enough. We know that such things sometimes last--and sometimes don't. And when they do last, this is not because they promised at the outset to do so. When two people achieve lasting happiness, this is not solely because they are great lovers but because they are also--I must put it crudely--good people; controlled, loyal, fair-minded, mutally adaptable people.
"We Have No 'Right to Happiness," God in the Dock: Essays on Theology and Ethics, ed. Walter Hooper, Grand Rapids, MI: William B. Eerdmans Publishing Company, 1970, p. 321