The fact is, that there remains a culture amongst many men (and women for that matter) that respects physical strength and fighting ability as desirable qualities in a man, and tied up in this whole way of thinking is, for example, an expectation that as a man, if someone hits you (or even insults you) it's a matter of pride to hit them back. In this kind of culture, men (or at least the stronger ones) are constantly on the lookout for opportunities to fight, in order to maintain their status. I do think that the "never hit a woman" taboo stems from this kind of culture.
In a culture where physical strength/fighting ability is not particularly respected, and violence is only tolerated as a last resort, there is no need for such a taboo.
I think the "never hit a woman" attitude is in most cases very much a learnt thing, and you'll mainly find it amongst people who have grown up in situations where vestiges of that "fighting culture" remain, either explicitly or under the surface, or simply where the taboo has been inherited without most of the rest of the culture. I think that's what annoys me about it - it's because it represents that kind of culture, one which I have very little time for.
(The reasons I have little time for it being, briefly, that determining hierarchy on the basis of strength seems like a foolish way to organise a society where physical strength isn't actually needed to do most of the really useful things, and also that the principle of retaliation just leads to a vicious cycle of lots of people getting hurt to no useful end. But I would assume those are fairly obvious reasons.)
I think that if you are indoctrinated into that kind of thinking when you're a young boy, I guess as a result of listening to older men, who you are easily impressed by, then the "never hit a woman" mantra is going to remain with you quite strongly even if you move away from that culture and even if you're the type of bloke who would normally do everything possible to avoid violence. And I think this explains why many perfectly intelligent and non-violent men find it hard to give a rational reason why hitting a woman is intrinsically worse than hitting a man beyond saying "it just is, OK?".
At the same time, sometimes the "fighting culture" attitude stays, often alongside an even more extreme insistence on the "never hit a woman" line. Normally only to be revealed on a High street somewhere on a Friday night. These are men who enjoy fighting but more importantly, do believe that their status in their own eyes as well as others' is determined by their ability to inflict injuries on another man. Even if they don't feel fully comfortable about this. And I bet there are some of these guys reading what I have written on this thread, and thinking to themselves that for wont of better words I'm just a wuss, and being mightily unimpressed by my suggestion that, say, if someone hits me, or spits on my sister, or whatever, then punching them isn't an appropriate response.
I will be the first to admit that I am lucky (on the whole) not to have grown up in a situation where I had to prove myself through physical fights, and where most of the men I was around as a kid had the same sort of view as I do now. Had I grown up in a slightly different situation then maybe I'd be here defending the "never hit women" line as staunchly as any.
So that is my bit of amateur cod-sociology for the evening. I don't know if it helps explain why I feel rather strongly about this whole thing, and my suspicion of those who are super-insistent about never hitting a woman. I hope it also explains that I understand that someone who takes this line can also be perfectly civilised (in terms of what I believe to be civilised) and not wandering about the place looking for a fight. Whether or not my analysis of why they have such an attitude is correct.