Generalization but women tend to more often act on emotion in a relationship than males and as the poster above me said, seem to expect more from their partner than vice versa. So I suppose two women may be more likely to lash out in a rage either psychologically or physically, thus domestic abuse being higher in women. I'd be interested to know how much is psychological abuse.
Men I'd assume would have more physic abuse. We do have the social rule of "don't hit a woman" ingrained into us, clearly some ignore this, but for two men in a relationship, there may be a higher chance that one or both wouldn't seem a physical confrontation with their partner as such a big deal than if one were female.