I don't think it's fair to say everyone is stupid (and I would say of course they are "ignorant", as they don't know - you didn't tell them, presumably - therefore made a judgemental call based on the information presented to them) just because they presumably associate a certain behavior (stereotypical of those who are gay) with something you are not. It happens. Just be clearer with people so they know where they stand, that's all. Not saying be someone else, just say, ... "I'm bi and I like you". 'bout the size of it. If I looked like a woman, for example, I wouldn't say people are stupid and ignorant just because they assumed I'm a woman, ... sure, it'd suck like hell, but that'd be my issue, and something I could resolve by trying to make it clearer that I'm one way or the other. (like wearing a blue bow... hah xD)

I'm most definitely not stupid, but if I saw someone acting in such a way that screamed "homosexual", yes, I would instantly suspect or assume them to be gay, ... that's just how it works. lol We're only human, ... we associate things to other things, and make decisions and unfortunately judgements, whether good or bad. I'm not gonna feel guilty for it, 'though I do appreciate that it sucks to be in your position. If there are people who claim to work differently, then fair play to them, I suppose they're better than I.

It is interesting, though, how a woman can act very masculin and most people just seem to assume tomboy (like I do) but they could quite easily be a lesbian or even bisexual.

Ah, just easier to make things clear so there's no misunderstanding, like with a lot of things in life. Best of luck finding someone, be it man or woman.

PS:
You are also assuming that this theory of your friend's is correct, when it could just as easily not be. It could be anything. It could be that you don't give off any of those "hey, I like you" signals to sorta let the woman (or man?) know. It could be that your possible lack of reciprocation (or noticeable amount thereof) leads them to, yes, jump to conclusion, in thinking that you are gay. (I suppose, psychologically, it could be easier - less of a kick to the self-esteem - than to assume that you just don't like them.)

(just a perspective from a straight male)

PPS
Crap, sorry, I've just seen the above post and see that you said you posted this ages ago and admitted to being confused and what-not. Hopefully, if anything, this post has been an interesting read that perhaps gets you thinking.