Agreed that the female-on-male violence double standard is the worst.
Is there really a rule that says you can't hit girls though? Because as I see it, it depends more on which kind of hit is being used.
1) When someone slaps you in the face with the palm of their hand, they're making a statement. They are letting you know that you have severely offened/angered them. This is more often used by females (on male and female), but it can also be done by males, like Shiro hitting Rin in the face in the first chapter. It is meant to hurt but not to injure; it is not an invitation to start a fight. Do not hit back.
2) When someone punches you in the face with their fist, they're picking a fight. Go ahead and hit back if you want. The only exception here is if a woman (or a man, for that matter) is punching you in defence because you cornered/grabbed her or something. But if a girl socks you in the jaw out of the blue, I certainly wouldn't blame you for returning the favor.