Eamar said:
It's a hangover from when women were considered the weaker, more delicate sex who needed to be protected.
In terms of domestic violence I would imagine it also has to do with disliking the idea of a (stereotypically) much larger, much physically stronger man taking out his anger on a smaller target who couldn't realistically fight back, plus there are connotations of a husband "owning" his wife and being allowed to do whatever he wants to her (bear in mind that until relatively recently it was largely believed that it wasn't possibly for a man to rape his partner, for example).
That wasn't just a belief, it was enshrined in the very legal definition of what rape was. - And the outcomes of modern rape trials statistically still show all the same biases that this archaic definition would imply...
(By it's original legal definition, you could only be raped if you were married, and by definition your husband could not rape you. - That's because the definition of rape was originally the theft of a man's exclusive conjugal rights... Meaning that, on top of every other insulting implication of that definition, to top it all off, the 'victim' of a rape, was the husband, and not the woman that had actually been raped...)
That said, as a woman I have always found the "never hit girls under any circumstances" rule insulting. If a woman starts a fight she shouldn't be able to waltz off without any consequences solely because of her gender. Not that it's something to be proud of, but I've been in fights with people of both genders. The man I fought with was perfectly justified in hitting me back, even if he was in the wrong overall (from my point of view, anyway

).
*standard disclaimer about how ideally no one should be hitting anyone in the first place*
I agree. The idea that you should be able to get away with violent behaviour without suffering any consequences simply because of gender alone is pretty messed up.
On the other hand, the old idea it was based on is reasonable enough in some regards...
Someone who is physically powerful shouldn't be using that advantage to abuse or intimidate those much weaker than they are. (The assumption that that's actually always true between men and women is another issue of course...)
An extreme example of this kind of thing is if an adult gets in a fight with a child...
Even if the child started it, and is being quite violent, it's still not really ok for an adult to just hit them back with no regard for the difference in strength...
If you are clearly more powerful, you have a duty to show some restraint towards those weaker than you. This should apply in ALL circumstances really, and for any kind of power (physical, mental, political, social, wealth... etc.)
But, all too often it's forgotten about, and instead you have those who can overpower others just doing as they please with little regard for others.
Not that this really has anything to do with men & women anymore, because the assumption that a man is always stronger than a woman is just... Not really a fair assumption...