I have another challenging question!!
If men and women are truly equal to one another, then why is it more controversial for a man to hit a woman than it is for a man to hit another man?
Because, in general (due to sexual dimorphism), men are significantly stronger than women. The potential for harm is stronger if a man hits a woman than if a woman hits a man. Simple as that. Also, domestic violence is a very common problem. Solve your marital problems by hitting your wife!
Granted it can really be read as, "It's more controversial for a strong person to beat a weak person." Bullies, domestic violence, etc. A significantly stronger woman beating up a man is, to me, just as bad as a man beating up a woman. It's not so much the beating that is the issue but the abuse of physical power over another human being.