Furthermore, I've had a number of conversations with girls who thought it was alright to hit guys.
It usually goes something like this. I'm on the thinner side at about 160lbs., so I use my weight in the argument. I usually ask, would you walk up to a dog that weighed 160lbs. and punch it in the head? No, because you'd probably get your face bitten off. Would you walk up to a 160lb. chimp in the zoo and punch it in the face? No, because again you'd probably get your face torn off. So, why would think you could punch a 160lb. man in the face and not get destroyed?
If they respond with the "because I'm a girl" nonsense I then try to explain to them that humans are animals... and among the animal kingdom, we humans are some of the most murderous, violent and aggressive animals on the planet, thus our dominance. It is dangerous and unwise to attack such an animal and not expect retaliation.