I think MMA fighters and other professional athletes have some sort of clause in their contract stating that they won't beat the shit out of people because it could be fatal. Don't quote me on that.
I also want to point out that slapping and punching are two different things.
At the core, the real issue here isn't hitting girls. It's the skewed view society has regarding a situation where a female being hit might be justified. It puts males in a compromising situation where they might face exaggerated consequences.
