You know how some couples joke about having a "free pass," i.e. the one person they would want to be allowed to cheat with if they met them someday? It's a common enough trope that it was in a Friends episode. You're supposed to name someone you would never actually have the chance of meeting like Tom Cruise or Nicole Kidman or something, because it assumes you're a decent person who doesn't really want to cheat on their partner. It's not something you really want to happen. If one person said "I want my free pass to be Brad Pitt" and the other person said "I want my free pass to be Nancy, my secretary," one is a joke and the other is a real possibility.

In western media, extreme violence is allowed because it is so extreme that it remains abstract. We can watch a graphic decapitation because we can remain distant from it. It's not something a decent audience member actually plans on doing.

Nudity and sex, however, is something real that the audience can relate to and be "turned on by"; it doesn't remain abstract. And that's why people can have more of personal reaction to it, and be offended by it. It makes them feel things they may not want to feel.

Side note: You ever watch that episode of Community where Dean Pelton watches a video of people dressed up as Dalmations having sex and says "I hope this doesn't awaken anything in me..."? It's like that.