That is what this chick seems to think, yet I think not.
Sure, a female MMO player is more likely to be subjected to whispers about her breast size, or jokes about her availability for dating or sex (surely a joke because we all know a girl playing an MMO is a fatty anyway). But why is this sexism? Male players have to deal with being called gay and having their ... sizes unfavorably compared(not cool, Skullgrinder Chasm players). This isn't deep seeded and corrupting sexism, this is just the common phenomenon of Internet users taking advantage of the wonderful, near total bomb throwing impunity that comes with anonymity. Note my expert use of this within my own blog.
More importantly, any MMO dev or player worth his salt knows that the stay at home or working mom is absolutely crucial to keeping the social side of the game running. The "hardcore" gamers come and leave an MMO within a month or two, but it is the stay at home mom that sticks around and keeps the game community (and thus the game itself) alive for years. If there really was soul crushing, ugly sexism running through MMORPGs, why would the stay at home moms be sticking around?
Plus, scientific research has shown that chick players get free stuff just for having boobs.
Sexism? More like sex-awesome.