Violence is unacceptable regardless of who its against. Nothing ever good comes of anything physical, it only has the potential in destroying your life or somebody elses. That may sound a touch dramatic, but its really not. You throw a punch these days and someone can easily pull a knife, or you throw a punch and they drop and they hit their head the wrong way and its disaster. Its not worth the trouble at the end of the day.
However
The reason why hitting women its frowned upon, or one of the reasons is because overall women are usually physically the weaker sex. I think that's what it boils down to. I was taught from a very early age that you don't hit women. Its an old school mentality that still exists today. Think it goes back to the old days when Men were the main breadwinners and it was their job to provide for and protect their wives. Talking about equality is fine, but you cannot include a defenseless woman getting slapped around by her bf who weighs 50kg's more than her as equal.
