The premises that a girl is getting paid for it, or that she's there of her own volition are moot. Hitting someone on set, or off, constitutes assault and is a criminal offense. I had a long discussion with my attorney about what can and can't be done on a film set; that's one of them.
There is a tremendous amount of misogyny in the industry today, far more than there ever was. As the industry grows ever wider without oversight, there's ever more room for abuse. Where will it end? I'm already hearing rumors of people shooting a series called, "Fucked to Death", in which a girl is violated so badly that she "dies" in the end.
Now that's sick shit.
I'd put scat right up there, too. Golden Showers have been around for ages. Thomas Pynchon wrote an exquisite passage in Gravity's Rainbow about a guy getting shit on by a German Dominatrix. Nothing in porn could touch it.
KN