The actresses were absolutely in on the slut shaming joke. Naomi and Charlize looked ashamed and upset while Jennifer gets to be proud she hasn't shown her stuff. That's because Seth is known to be offensive. Seth was indeed self aware when he offered that "what did you think you were getting?" bit after a joke was booed. In fact, four characters that are sexually assaulted and/or raped in said boob-showing scenes, according to this. I understand laughing at something like the boob song. Seth's shock humour seems designed to make you laugh at the absurdity of it. But that doesn't mean what he says is actually funny or acceptable.