female avatars is ancient in gaming, from the Queen chess piece on, as an early example. gender and race being different than your own is par for the course in games. whole hobbies circulate on the assumption of wearing different masks, and exploring that, like roleplaying games and simming.
it's been there from the start of anthropomorphized video game characters, too. honestly i can't remember anyone, besides easily weirded-out pre-teen children, freaking out over Ms. Pac-Man, Samus from Metroid, Alis from Phantasy Star, Blaze from Streets of Rage, and on and on. growing up on arcades no one (openly) freaked out over a male playing Storm or Dazzler, or Marge or Lisa, on four-player X-Men or Simpsons, or Chun Li or Cammy in Street Fighter. (and females who grew up playing video games are quite familiar with playing male avatars without anyone batting an eye.)
MMOs don't change anything about this dynamic except the humanized element that the avatar talks back.
it's a pre-teen child. in those horrible awkward years. who probably doesn't get out enough.
don't over think it and relax.