Wow, first off might I say that of the comments I've read some really interesting stuff has already come up in this thread.
Ok, now for my tu'penny worth:
I'm stright. In fact, I'm so stright I've never felt inclined to even label myself as that. I think that's a good way for all of us to be. Unlabeled. I've know people over the years, some stright, some gay, some claiming to be undecided (I circle back to that one), although I don't think I personally know an out-and-out bisexual individual (I could be wrong, I'm not usually one to ask!).
Of this broad spectrum of people I can say that no one person has felt like a 'stereotypical' example of their sexual orentation. I know masculine homosexual men, afeminate stright men, lesbians I didn't even realise where gay (proof that you are not your sexual orentation!) so I think to say you are either/or is such a simplification that it makes the whole thing meaningless because people aren't the same!
Now onto my opinion of bisexuality. Bisexuality is not, 'undecided'. In a simpler time when I was much, much younger than I am now I thought bisexuality was being undecided but it is clear that it's not something you decide upon. I believe sexuality is nature not nurture (9 times out of 10) so being undecided is when a person has not made a decision about if they are stright/bi/gay or anything inbetween.
The notion of bisexuality is a tricky one for me to comprehend not because I think that it is somehow 'wrong' or 'not real' but because I wonder in practice what it means for the individual. Would they only date other bisexuals? Do people not always realise they're bisexual stright away? What if they dated someone stright and they then broke up and dated someone of the oposite sex, would the stright member of the relationship feel that they were never loved?
I realise that each bisexual is as different and individual as each stright or gay person but those are particular questions that play on my mind when this topic comes up.
There defiantely needs to be more understanding about sexual orentation so there can be less prejadice against people of all orentations.
In the end, people aren't a sexuality, they're people, wonderful, unique people, and trying to put people in boxes often causes harm not good.