One claim I've seen is that men commit more violent crime because it's more common for a man to be physically stronger than another person, while the rates of non-physial violence are much closer across gender boundaries.
It might also be non-linear. A person who has half the strength of their partner might be significant less likely to be the attacker than if a person has just 10% less physical strength. It would be an interesting read.
It seems that the only thing that changes with gender is that when there is a conflict and it "eventually" develops into physical attacks, which is a strange concept in itself, the person who will be the attacker (or at least be found guilty in courts) will be significantly more likely to be a man if the relationship is heterosexual.
Come again? What's non-physical violence? Mental violence? Writing someone a mean letter?
It can be voluntary or not but dismissing it like that seems strange to me. It's not because it doesn't leave physical scars that it can't cause trauma.