I feel that sex is a pretty important attribute. Even if the character could function the exact same way as the opposite sex, they won't likely be treated the same way by the other characters. So while the character may not change, their interactions with other characters will.

The main difference I see between the sexes are how they are perceived by society and the reader. [@The Harbinger of Ferocity] and [@Mr Allen J] touched on the former. I like to use double standards as a tool to draw attention to what a character is doing. No one would bat an eye at a man leering at girls at the dance club. We'd write him off as a perv and be done with it. But what if it was a woman looking at a group of dudes?

I feel that balancing the sexes is a good idea, because it creates diversity. But I do agree with what everyone else brought up. a magical girl squad won't have any men in it, and a WW2 themed RP will contain all dudes most likely. This kind of ties into certain sexes being better at certain rolls.