Women tend to be smaller and less muscley than men so we usually have very little chance of defending ourselves against a physical attack by a man. This is why people tend to get particularly upset when a man hits a woman. It's not because they think that hitting a man is OK - they just think it's especiallly vile when the victim has little chance of defending themselves. Pick on someone your own size etc etc.
If this is the reasoning, then people should say "I would never hit someone weaker than myself" instead of "I would never hit a woman".
The whole "I would never hit a woman thing" irritates me because by implication it means that sometimes it's OK to hit a man. Well, if there are situations where it is OK to hit a man, then there must be situations where it is OK to hit a woman.
Firstly, not all men are stronger/bigger than all women. They tend to be, but that doesn't mean there is never a situation where a woman is physically stronger than a man.
Secondly this whole attitude seems to suggest that relative strength or size are the only factors in someone being able to defend themselves. Well, they aren't. So making a judgement on whether someone can defend themselves based entirely on their size is flawed in the first place.
Thirdly: why should someone's ability to defend themselves be all that relevant anyway? This all assumes that they will fight back in the first place, and not everyone will. Even if I was large, strong and adept at fisticuffs I wouldn't want someone using that as a reason to excuse injuring me. Besides, if you are going to stick to the logic of "never hitting anyone weaker than you", then anyone bigger than you is prevented by these rules from hitting you back, therefore giving you an unfair advantage.
Fourthly there are lots of other factors which affect a judgement of whether or not hitting someone is OK, including for example provocation. If we are to stick to a strict "never hit women" (or even "never hit people smaller than you") rule we end up with nonsense situations. What is "worse": a small woman injuring a large man entirely without provocation, or a large man injuring a small woman having been deliberately and heavily provoked? I would say that the latter is "worse" but the silly "never hit a woman" rule would not allow this conclusion.
Personally I don't think it's ever OK to injure another person unless necessary in self defence or you have some kind of agreement between the two of you that you are both happy to engage in physical violence. Just before anyone pipes up saying that I am happy to hit women.