I *guess* the appeal for non-religious folks would be similar to the appeal of nuns or librarians: taking a character trope that is essentially non-sexual and making it sexual. It's a perverse process by definition (and I don't mean "perverse" in a bad sense, just in its technical sense), and like other people who commented before me, I don't buy the "They're religious, so it's okay" argument. Even if the books themselves are tame and sanitized, the process is still what it is.
no subject