Men, being stronger and larger than women, are -- quite correctly -- usually taught that while violence in general is bad, hitting women (and children) is particularly to be frowned upon as a cowardly and unmanly act. There are of course exceptions, life and death situations -- say, a woman is going to attack you with a gun, etc. -- but I don't think that's the point of Eyeron's question.
His point is this, I think: there are many daily-occurring situations where a man probably would hit another man. Do you think that, in some of those situations, hitting a woman would be justifiable? The old gentlemany answer was, "no, never". I don't think that's bad. I think it is in fact better than the claimed "it won't matter to me if it's a man or a woman" indifference expressed by some people here. I don't think the people here who are shouting for "equality" in the treatment of men and women in this case thought it through.
Suppose you are a man, and a woman shouted at you with "fighting words", telling you what a worthless piece of scum you are. Or suppose a woman slapped, or attacked you with her fists, or threw her drink in your face. If a man did that, you'd hit them back, probably trying to knock them out. But if a woman did that, I hope and trust most men here would control themselves and not do that. They would perhaps try to restrain the woman from hitting them again (say, by holding her hands), but I trust they won't punch her in the face and knock her out.
What's more, I would be deeply suspicious of someone whom I see punching a woman and knocking her flat, and then saying, "well, she attacked me first, and if she were a man you wouldn't think it's wrong". Even if -- technically -- they are correct, I would think such a man as cowardly, of not acting like men should. Men should, within reason, prefer not hitting back women even if they are hit first, and should attempt to restrain, not hit back, their assailant.
(Again, there ARE of course exceptions and extreme cases -- I am presuming we are not dealing with extreme circumstances where punching the woman is the only way to avoid grave bodily harm to yourself or others, etc. I mean in the typical case.)