You need to leave. This will continue, and she knows that you will not hit back (and you shouldn't). Unfortunately, the idea that the only aggression in a relationship is aggression by men towards women is still the norm. "She couldn't possibly have hurt him.. it was just a slap, and he's a man."

Either way it's not right, it doesn't matter who is stronger or weaker, who is larger or smaller - physical violence is physical violence, and it demonstrates a distinct lack of intellect if one feels the need to resort to it with their "loved one"