I don't even understand how anything of that form could be right -- because there are potentially relevant differences between drinking alcohol and committing murder.

Of course there are differences. But the differences lead into my other objection, which is that, as the old joke goes, now we're just arguing about the price. If he supports laws against murder because murder does a lot of harm compared to compulsion and stopping it has few side effects, then if he were to be convinced that divorce does a lot of harm and stopping it has few side effects, he would support laws aganst divorce.

Then consider why Lewis believes that divorce (etc.) is harmful. It's arbitrary--if his religion had said something else, he'd have believed something else. And likewise, his belief in the degree of harm done by divorce is arbitrary. His religion happened not to say that that particular sin was harmful enough to justify banning. But it could have said that. And given a long list of sins, it would be a pretty big coincidence if it didn't say that for at least one of them, just by chance.

(I suppose there's another possibility: Lewis doesn't want his religion to tell him something is bad enough to ban. His interpretation of his own religion is biased by this desire, so he'll always interpret his own religion as saying that a sin isn't bad enough to ban. In that case, I need not fear Lewis banning anything. I guess that's a defense of Lewis, but I would then note that this kind of bias seems to be pretty rare among religious believers who don't like divorce, gay marriage, etc.)