I don't think I'd hit back if a woman hit me, but I still think that whole 'you can't hit women' thing is ridiculous.
Edit: Has anyone said "With great power equal rights comes great responsibility" yet?
Edit 2: If the woman attacking me is smaller than me I think I would go for restraining her rather than punching back. I don't know why. I'd certainly hit a smaller guy if he punched me. I guess it is just some primal part of my brain that says 'potential sex partner, don't hurt it', and then I instinctively will try to get through it causing as little damage as possible to the woman. Guys don't have much value to me in that sense; hell, they're rather potential rivals, so if I break them, that doesn't matter much to the primitive part of my brain, where I am pretty certain my fighting instinct lies.