Seriously, the "don't hit girls" rule does not apply in society anymore.
EDIT: An explanation to my thought.
Back in the day, when women did not have rights, it was considered "unlady like" to fight. If a woman was unlady like they would not be able to get a husband, which was basically the only way they could survive in that old society. So, if you hit a girl, they could not fight back. This is my assumption on why men were not supposed to hit girls.
Now, since women can survive on their own, this rule does not apply as much. Obviously, you shouldn't beat the piss out of girls. But that doesn't mean they have immunity either...