My daughter is now 7, so this is no longer an issue, but I would never let her be aggressive (slap, bite, etc.) toward another child, boy or girl. I definitely let kids roughhouse as long as all kids felt it was a game, not one child attacking another, but whatever rules were in place applied to all.
The behavior I don't like, is not based on the sex of the child, but the sex of the parent :) There have been numerous times where a dad is in charge of "watching" his child on the beach (we're talking about kids 2 and under here) and as soon as the little boy started to "play" with my daughter, the dad would be 50 feet away, completely engrossed in conversation with his buddies, not watching his kid at all. The kid would have come over with no toys to "trade/share", grab my daughter's toys that she was playing with, or take handfulls of wet sand from the puddle she was in and start throwing them at her. First of all, I would never "abandon" my daughter to someone else's care that I've never met or seen before without so much as a "hello", 2nd I don't want to have to discipline someone's child I don't know, 3rd, I'm not your babysitter, what if I was about to get up and take my kid in the ocean, and your kid wanders off. don't get me wrong, there are plenty of attentive dads on the beach, but it was NEVER a mom that left her son or daughter unattended near me.