Nearly every VRMMO game in the series randomly generates avatar appearance. In most games, all you can choose is your name, race and maybe modify your hairstyle later on. Asuka Empire might be the only known game with more customisation options, but even then it apparently doesn't allow full control over avatar customisation.
In-universe-wise, Kirito just happened to get a very rare avatar. Though this of course has use for the story too. Kirito wouldn't have become acquainted with Sinon if he looked like a guy.
Athcat3692 wrote: Personally, I think the avatar randomizer is a stupid feature. Avatars mean a lot to people and taking away the element of choice would realisticly cause a huge uproar.
it would also provide a large risk concerning security - refer to S1E1. two players are tricked into believing the other's entirely false avatar, one pretending to be female and the other 17 - something that goes on alot in many places anyway, but would be much more effective in VRMMOs
It is rumored the "M-9000"-type avatars, which create the appearance of an F-class avatar, can be gained by converting a VRMMO account with extremely high playtime, which most likely explains it due to Kirito's time in SAO, then ALO.