Inside every one of us is a certain "code"--a mental list of rights and wrongs. Of course this list is different for everybody, but when you see or hear something you feel is wrong, there's no doubt about it.
In literature, I understand what somebody feels is morally wrong might be okay fodder for someone else to include in a story. Heck, I wrote 2 books about women whose husbands had cheated, and each of them reacted differently in response. Even the one who didn't retaliate (though she yearned to) offended a few old biddies in a certain writing group. Well, whatever. My point in that book was to demonstrate the black/white/gray in all of us, and that it varies from person to person. Sometimes watching a movie/reading a book where the characters do something controversial can really get you thinking about how you'd handle a particular situation (thinking of a movie here, can't remember the title--where the rich guy offers the couple a cool million if he can spend a night with the wife). These stories get the wheels turning, and make you see different sides of the coin. Maybe even the edges!
But there are lines we don't want to cross.
I recently reviewed a submission where the heroine cheats on her fiance while she's out of town. And gets in a huff when he shows up on her trip and suspects something. And never tells him! Yeah, wasn't liking that character very much. That's one of those "lines" in my mind. Now, if the character's actions are led up to with sympathetic actions and feelings, and there's a reason behind what they do, I can still read something like this. Doesn't mean I'll condone the actions, but it might be worth a read. However, if she does this just because she "can"? Big no from me, and I can't see reviewers liking it either.
Got another submission where the hero murders people who get in the way of what he wants. Thanks to synopses, I never cracked that one open.
Someone else might like those stories--I don't know. Both of them are so far across my "lines" that I don't really care how good the writing is.
Have you seen a movie or read a book where the main characters did something too "bad" for you?
Romance is sexy!