Depends on the force of the hit. If you are in danger, as in the woman is coming at you in a violent manner, or being aggressive. Of course you need to defend yourself. You aren't just going to sit there and let her hit you, right? I hope not. Equality comes in all forms, and not all woman are physically weaker than every man. You don't need to "knock the bitch out." So to speak, you just need to use the right amount of force to settle the situation down.
However, this goes hand in hand, of who is physically stronger. If she is obviously going to do some real damage, then you have every damn right to defend yourself as much as if it was another man attacking you. It just varies on the situation, how much as it escalated and so on and so forth. For example, if she was coming at me with a knife or something ridiculous like that, of course I'd use enough force to end it right there and now. If your morals go over your own personal safety, you may need to sort that out.