The only reason its seen as "bad" to hit a woman is because of long standing social conditioning stemming largely from ye olde chivalry. Women were precious little roses that had to be coddled and looked after by the big strong man and only an uncouth savage would strike such a helpless creature. This is a "western" thing for the most part. All you have to do is look at some Asian or Middle Eastern cultures to see the difference in outlook.
Nowadays, in this magical era of gender equality, women are by in large expected to look after themselves. Sure, there is still a cultural, if not instinctual, imperative amongst men to protect women, but for the most part, men and women are equals. Part of that equality means being held accountable for ones own actions. If a man strikes a woman, yes, he is in the wrong and the woman is well withing her rights to retaliate. Now, given the general difference in strength between males and females, it is less likely that the woman would be able to inflict much in the way of damage unless she knew what she was doing, but she has the right to try.
However.
If the roles were reversed and it is the woman initiating aggression against the man, he also has the right to retaliate. Now, because of cultural conditioning and instinctive nature, most men would not retaliate. That does not mean that he has to stand there and take the abuse. He is making a conscious choice not to hurt her. Not every guy is willing to make that same choice. Bear this in mind ladies the next time you think its okay to strike your man.
Rule number one in not getting your ass beat - Keep your hands to yourself. If you throw that punch or slap, be prepared to suffer the consequences. Because not everyone is going to just stand there passively and take it. This goes for men and women.