Why can't a guy retaliate against his abusive wife or girlfriend?

as hard as it is for anybody in the situation, male or female, hetero or same-sex relationships, to be blunt you leave, you walk away and don't look back, because the situation may change temporarily when they feel they are losing you but when you go back the situation goes back to the way was and becomes more difficult and quite possibly more abusive, and if you retaliate you possibly risk being arrested for domestic abuse even if they started it. 

Spousal abuse is wrong~ from either spouse; not just a man beating on a woman. Even though that is the majority of the abuse cases, men hitting and hurting women, I do understand there is a small amount of relationships where the abuse is reversed, but society has chosen to not acknowledge that fact, due to not understanding that a woman CAN cause physical damage to a man. That being said, and this is my personal opinion, nothing else, but if a man is being beaten to the place where he thinks his life is in danger, then do what ever it takes to get away from the immediate danger. It it means hitting, so be it. And I will say the same thing regarding a woman being in the same situation. Do what ever it takes. But~~ once you are out of the immediate danger, get away, stay away and don't go back. Ever. So many make the mistake of thinking that the situation can be worked out; maybe it can, but why risk it? A life is not worth a failed relationship.

He can, by calling the police like any normal person would do.

Retaliate is a bad idea. React is a better idea and if there is real abuse the reaction might be to end the relationship or to give her an ultimatum to end the abuse or you will end the relationship.