Twenty years ago i would have agreed with you. But these days when you see a fight like that, its more than likely the woman started it.
In these cases, if the woman started it, and its obvious she has the capability to hurt you, then shes fair game.

I have seen a couple go that way myself in the past 10 years, even saw a woman knock out a guy with a sucker one punch, no she wasnt that big and he was average size.
If you know how to punch, even if your an average build girl, you can really hurt someone, just ask Ronda.
A friend of mine, she is so softly spoken, so nice and sweet, 5 foot 6, around 130 pounds, but if she got mugged in a dark alley by 2 guys, i would fear for their lives.

On the other hand, i also saw a guy slap his girlfriend pretty hard for nothing more than spilling his beer. When me and my mate tried to get involved, not only did his girlfriend come to his defense, but so did half the bar.