I think if a girl hits you then most of the time you should man the **** up and just walk away. On average I can't imagine a girl being able to hurt you that much, theres no reason to swing back. To me its the same principle as if a child hit you.
I'm absolutely not speaking down to women by this; I just don't know a lot of girls that could hit hard enough for it to do damage or require some sort of self defense.