Board Thread:News and Announcements/@comment-6697950-20140102180802/@comment-6697950-20140419092639

The game automatically picks an avatar (including gender of the avatar, as the gender is fixed to be the same as the player's real life gender) for the player based on their brainwaves. Kirito was one of the very few people with odd brainwaves and the system could not decide which gender Kirito belonged to, thus it ended up giving him a very feminine, but still male, avatar, the M9000 type. Of course, the M9000 type looks as feminine as actual female avatars (some guy even mistook it for a rare F1300 female avatar) and is worth quite a lot of money.

Updated the OP with the question. Can't believe I forgot to add it before...