Well, keep in mind it all stems from basic chivalry, which was put into use to keep guys from unfairly picking bar fights with chicks... (kinda)
However, if they hit you and say, "Don't hit me back, I'm a girl," you then have every right to treat her as men would back in the day and smack her ass. If she hit you pretty good, you then have the right to smack her ass, but then at the end of the swing, take one cheek in your hand and jiggle it.
If she hits you in the crotch, you may do all of the above, and growl, "Ooh, you like it rough, huh?"
Please note that all of this is a joke and not to be taken seriously. Sgt. Pepper is not responsible for any injuries/lawsuits resulting from taking this advice.