Well, I don't agree with your last point - I know a lot of people, especially a lot of bi people, say that everyone's a bit bisexual, but I don't think so. I think it's a spectrum, and all points on that spectrum are as valid as any other. 'Bisexual' is just a term for people more in the middle of the spectrum than at the edges.
But yeah, I don't know why people think it's so hard to understand either. Or, actually, why it matters if they can't understand it. I don't understand being straight or gay - I mean, I understand what it means and I accept it and everything, but it doesn't make *sense* to me like I can't imagine feeling it, can't quite get my head round how it must feel to only be sexually attracted to one gender. But that doesn't mean that I can't accept that people feel it, and that it's totally valid and reasonable. I don't understand why straight and gay people can't do the same with bisexuals. And you'd think that homosexual people would be better at it, but they're just a little bit quieter about their prejudice. Only some of them, of course, and some straight people are great, but it really fucking hurts when you think that the gay community will be a source of strength to you and then it's not.
And then there's the thing where if there isn't any open prejudice against bisexuals, they get ignored or left out of LGBT things. Like the letter is included, but say you're at a talk or a conference or something the speakers invariably speak as if everyone in the room was gay, with no recognition of bisexuality. It's often the same for transgender issues - there's the assumption that everyone in the room is gender normative, it sucks.
Sorry for the rant xD I'm usually alright with it, like I just shut up and get on with it, but I guess last night upset me a bit and now I need to let it out xD