I voted in your poll when it was new, thinking it was a slightly odd thing to ask, but having thought about it, I decided to come back and elaborate.
I am a human of the female persuasion, but I often take advantage of "no girls on the internet" syndrome and allow people to assume I'm male. If you claim otherwise, one of two things tends to happen:
1) "Wow, really? And you like the same games as me? Pics please?" Which can devolve into "You're not REALLY a girl unless you send me timestamped pictures of your breasts, WHY WON'T YOU?"
2) "Go away, we don't need any attention whores here."
I'm not a woman to get attention, I play games because I like playing games, not because I want in your pants. The same is true for most girl gamers I know. The ones who want in your pants aren't usually that interested in the game itself - they're playing a whole different game. There's a few ways to avoid this.
1) Don't admit to being female. Ever. Even if the topic comes up, if you don't know the people you're playing with, let them think you're a guy. Does not work in real life, or anything that requires you to talk over a headset.
2) Only play games no one is "surprised" to find girls playing.
3) Attempt to politely deal with and correct people's assumptions about you.
Most of the girl gamers my age (late teens/early twenties) that I know use strategy number one. It's like a general internet safety rule - you can do whatever you like, as long as they think you're a guy.
Most of the older girl gamers I know use strategy two. Or maybe they actually just only like "girly" games, I haven't conducted any research on the topic.
Strategy three is probably the best one in terms of helping get rid of the problem, but it's a lot of work when I just wanted to play games, not engage in feminist debate.
Anyhow, I suppose my response to OP is, no, you don't have to know. There've been times, I guarantee it, where you DIDN'T know. And if you hang out where I hang out, it'll happen again.
That said, most of the Bay12ers seem to be pretty reasonable people, so I don't mind "decloaking" to add my two cents.