I think there's a real selfish mindset to it: people who do this like using their sexuality to get a reaction, even if others don't enjoy it. They like "making a man hot under the collar" or watching someone try to decide whether to say something, or then feeling superior because they didn't name the violation of social norms.

I kinda get the other side: if we were more open about sex and nudity it'd save a lot of grief. But I think you can tell when someone wants to do that versus when they get a power trip from sitting around with their tits out, because it's done in smaller ways.