Why is hitting a girl wrong?

According to some made-up law girls can hit girls, girls can hit guys, guys can hit guys but guys can't hit girls. Why not? IMO, anyone who thinks hitting girls is wrong is a sexist because they are giving girls different rights to guys just because their girls. Also, if it's because guys are supposed to be naturally stronger than girls then the girl shouldn't pick a fight with the guy should she? BTW, when I say 'why it is wrong for a guy to hit a girl' I mean I defense, I don't mean for no reason