I actually was attacked by a female friend of mine a couple of years ago. I'd upset her the night before by rejecting her advances, and the next morning she burst into my hotel room and started swinging her fists.
And I didn't fight back. I don't know whether that was because I thought of her as a friend, or whether it was because she was a girl, but I tried to talk to her and calm her down, whilst trying to hold her back.
Which in retrospect was a mistake, as she took the chance when my hands were tied up holding hers back to kick me full force in the balls. Hard enough that I passed out...
We have not spoken since.
Honestly, despite that I'd probably do the same in the situation again, I'm just not a violent person and if I can stop a fight without swinging a punch I will. If there was no stopping her though I would fight back, that's just common sense.