Why would you intentionally build a MOBA with imbalances that are literally not only a planned, but an advertised feature of a game?
The MOBAs that survive are designed to be competitive. That is, your gender gives you no advantage or disadvantage. It all relies on the skill of the individual, the skill of the team, the team composition and, most importantly, the team's ability to work together.
It can have a bunch of female players, but if it's inherently imbalanced the game just won't survive in the current turbulent waters of the MOBA genre. Considering the plethora of free and balanced alternatives, if someone has a team of mixed gender and it gives them a bullshit advantage, other players are going to call bullshit (which is fair) and just move on.
Likewise, this seems like it will just increase harassment from all the idiots out there. Not only do women currently get harassed online for no reason (besides that 13 year old kids think they're cool for doing it) now they can't even hide behind anonymity to disguise their gender. I don't think they should be NEEDING to do this in the first place, but it's an option when you're fed up with the bullshit.
Now you can't. And not only can you not, you have a "reason" to be the target of verbal assult from your opponents in that you're buffing your team and a lot of people are probably going to chide you over it.
This is just a bad idea.
EDIT: Could it just mean female and male CHARACTERS mixed? If the team benefits from a hero/champion male and female mix that's a whole other thing. It's kind of stupid, but it's not that bad as I believe men can still unlock female heroes and vice versa.