You are mistaking gender with sex. You can only have one of 2 sexes, gender you can pick your flavor.Not to talk shit on the 108 genders that society has now, but if you are born with a dick then you are a guy and if you are born with a vagina then you're a girl.
As someone who used to go with female characters on mmos, my motives were simple: male armors and appearance on most mmos are as best generic and at worst horrendous. If im going to spend hours looking at it, i prefeer it looks good.That's why I find it weird when some people use the opposite gender when going online. I understand why they do that but I still find it weird
That and it also do the BS thing that happens on SAO. It scans the face and i dont remember if it scans the heigh or if the person have to enter their height on start. On one way or another it get some info of the physical body of the user too.I don't think it was explained at the start of the manga, but this VR device scans your brainwaves to determine your gender. But Shun is so feminine and has mannerisms of a housewife that he managed to trick the system into giving him a female avatar.