He wasn't obsessed with it, as a reader reminds me:
Note one important corollary [about Lewis' distinction between civil and religious marriage]: Lewis wouldn't have regarded such a distinction as permissible if he thought that "non-Christian marriage" and relatively easy divorce was a really serious sin, any more than he regarded, say, murder or thievery as morally permissible for non-Christians. The same thing is true of his attitude toward homosexuality - as far as I can determine, he mildly disapproved of it but was simply too morally sane to regard it as a serious sin:
"I have never been able to understand how sexually normal people can regard homosexuals with anything other than a kind of bewildered pity."
We don't need the pity, but this is certainly infinitely more agreeable than self-righteous hatred. And in the chapter of his autobiography dealing with the year he spent in a horrendous private school that he calls "Belsen", he notes that the top-ranked bullies had an accompanying set of catamites - and then talks at some length about how this was the only sign of genuine human affection that existed in the place, and points out that there are infinitely worse sins. (He wrote this in the Britain of 1958.)