It's true that Lewis separated religious and secular law, but presumably Lewis would want laws against, for instance, murder. It's hard to consistently believe that we should have laws against harmful things, have a skewed idea of what constitutes "harmful things", and not want laws against them.

One possible response is that the harmful things only harm oneself, but Lewis believed that such things harm society, not just oneself. Another possible response is that as a practical matter, it would be a bad idea to ban such things, but that only lasts as long as it's practical--such principles would not lead to the conclusion "we should not ban gay marriage" but rather "we should only ban gay marriage if we can get away with it".