Oh, so guys cant hit girls but girls can hit guys...hypocracy.

Anywho-I always thought, story-wise, that Haku *became* the female persona to Zabuza's male persona due to Haku's trauma as a child. He wanted to be comforting and nice to be accepted (women are, supposedly, the more loving gender). This way, Haku can create the love
that he's always wanted, by *being* the gender that is suppose to, traditionally, give it while Zabuza gives the traditional fatherly discipline.
Though I am not advocating that fathers are necessarily more violent on children, mothers are just not expected
to do that to their children moreso.