The game is predominantly male, that is a fact. Chances are they wont believe you anyway and think you are a G.I.R.L (guy in real life), its not malicious its how things are. Believe me, if people began referring to others as hey girl, shit would get bad, really bad. Not only for the other guy being called a girl (which they can get pretty pissed about) but for girls too because it would just sound creepy.
Chances are they would treat you better anyway if they didn't think you were a girl. Yes its bad, but it cannot be helped. I dare you to try being a man trying to do something that is associated with the female gender, its pretty much the same thing, only sometimes worse because they don't have the support the female persuasion does when attempting to do things that males generally do.
What I am about to say is not malicious in any way, merely some words of advice.
Do not be offended when you are called the wrong pronoun. It is on you to choose what offends you, and if you want to take offense to people using the wrong pronoun, gaming, or the internet for that matter, might not be the place for you. For your sake, and for the sake of enjoying the game, suck it up. If I have to deal with being called %%%got every two seconds by someone I dont know, or will ever know, you can deal with being called the wrong pronoun, and you know they dont mean it with any malicious intent, but tell them your a girl, and it might turn their intent to mischievous.