Sorry to go all serious here and this is gonna sound really pretentious, but stick with it..
We all need labels and language to understand ourselves, each other and the world around us. Labels help us describe and communicate a variety of things. But labels are paradoxical i think, because as well as allowing us to name things and each other, they simultaneously allow us to limit our understandings of individual preferences.
Everyone's an individual and it seems absurd in a way that with all the millions of people in the world we have 3 defined categories in realtion to sexuality; gay, straight and bisexual!?!? I think everyone has their own individual sexual preferences which are determined by personality, genetics, childhood, upbringing etc - a variety of factors determine how we feel about people sexually.
I think sexuality is a bit like ice cream, there's lots of different flavours out there..
I'm really against people being cynical towards other people's definitions of who they are. If someone says that they are bisexual then i think this should be accepted and respected. I'm completely against bisexuality being sneered at, even if someone is confused, or is even in denial about being gay or straight, i think it's cruel, particularly if someone is struggling with something!?! to poke fun at them, sneer and completely disregard their sense of self.
We'll only be accepting of other people when we are truly accepting of ourselves and I feel that a lot of this anti-bisexuality feeling is really to with the people doing the sneering. Maybe bisexuality reminds the sneerers of what it felt like when they were unsure of who they were..