I know quite a lot of girls who keep fish but I know some of them have unisex usernames and avatars so they don't get sleazy messages from guys.
I guess they want to be seen purely as people who keep fish, rather then "a chick with a tank". You gotta admit it even on this site if a girl has a pretty picture on her avatar she'll get loads of flirty attention/pm's etc and while some girls are ok with that others aren't.
The most knowledgable fish keeper I know is a female and also my LFS has 2 females from a team of 7 so I don't think thats a bad ratio.
As for the women who were previously mentioned who are assumed to be, "dumb/non educated fish keepers" when they go to their lfs... I think if you speak to most people who work in lfs's they actually assume the majority of their customers who come in are uneducated fish keepers (regardless of their sex). I think in their line of work its presumed safer to assume ignorace as opposed to assuming most customers have a deeper indepth knowledge (we must remember most of their customers see keeping fish as some sort of equivalent or upgrade to keeping a pet hamster or similar)