“My dear L.,” Tolkien writes in a draft letter from 1943. “I have been reading your booklet Christian Behavior. I have never felt happy about your view of Christian ‘policy’ with regard to divorce.” Tolkien did not send this letter to his friend, C.S. Lewis, but it was found and published after his death.
[Y]ou observe that you are really committed (with the Christian Church as a whole) to the view that Christian marriage—monogamous, permanent, rigidly “faithful”—is in fact the truth about sexual behavior for all humanity: this is the only road of total health (including sex in its proper place) for all men and women. That it is dissonant with men’s present sex-psychology does not disprove this, as you see: “I think it is the instinct that has gone wrong,” you say. Indeed if this were not so, it would be an intolerable injustice to impose permanent monogamy even on Christians.
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Toleration of divorce—if a Christian does tolerate it—is toleration of a human abuse, which it requires special local and temporary circumstances to justify (as does the toleration of usury)—if indeed either divorce or genuine usury should be tolerated at all, as a matter of expedient policy.
Jake Meador discusses this disagreement more in his article for Christianity Today.