@Krystal
In many rural and traditional parts of the world, there is a culture of machismo. A man is supposed to be strong and in control. It is his manly duty to put women "in their place" either through physical or emotional coercion. If a woman is independent, free spirited and not subdued by a man, then that man loses his machismo.
The notion that it is un-manly to raise hand on a woman came about during the feminist era as a way to counter the culture of machismo. Ethical arguments against violence, coercion worked only in certain societies. In society where a man has to be macho, a man would rather be violent than un-manly. That is why the notion was spread that it is unmanly to raise a hand on a woman. Even then the notion is still that a woman is weaker than a man. He has physical power over her. And only he can protect and care for her. Encouraging women to stand up for themselves and even lash out with slaps or other physical assertion also was a means to counter male dominance. This was an issue where flight could not solve problems, fight was necessary. Especially for bruised or battered women.
Ideally, neither gender ought to stronger or weaker. Society should not run on either machismo or feminism, but a balance between both genders. Violence against anyone is wrong. Depending on the circumstance it is Ok for men and women to use physical force against men or women. Unfortunately, our society is far from being ideal. The best we can do is avoid stringent notions and try to evaluate each situation on its merit.