I have my own thought about it, too.
Being a nerd/geek (the two terms are often used interchangeable) was predominantly a male affiliation, and one that often came along with being socially ostracized, bullied, etc. It became its own subculture, with its own ethos and rites of passage. Any women who were part of it were generally expected to have come up the same way: socially awkward, few friends, little socialization outside the "geek" circle. Her gender was essentially irrelevant.
That's not really the situation anymore, though. Being a geek is mainstream now. It's cool. But you still have the guys holding onto this nostalgic idea of what it is to be a geek, and what you have to go through to be one, and they don't like that just "anyone" can be one now. This is a status that's supposed to be reserved for the brilliant yet misunderstood, and instead you've got attractive young women who play video games and read 4chan and call themselves geeks. Some people can't handle that, because they don't like their subculture being "invaded" or "taken over" by people who haven't "paid their dues."
I'm not saying I agree with them at all, because you just can't control a subculture in that way. I can understand where they are coming from, though. However, I think it is incorrect to assume the motive is sexist. I don't think sexism enters into it. It's more like a fear of something different, and especially the fear of these "different" people coming into your subculture and making it into something else, and then what have you got left? There are people whose lives basically revolve around living in geek/nerd culture, and once that is changed into something unfamiliar, it's like they don't have a "home" anymore.
In my opinion, the answer is for those people to grow up and realize nothing stays the same, and to get over their fucking childhood trauma already.
