A lot of people I know don't believe in bi-sexuality and I have always tried to figure out my thoughts on it because I am really conflicted. For me, and a lot of other young gay people, the first step in coming out usually revolves around coming out as being 'bi-sexual'. This, for obvious reasons, has given the term 'bi-sexual' a questionable definition because so many people who come out as being bi offered end up being attracted to only one gender. I do however believe there are people who are genuinely attracted to both genders, and I know of people who have had sex (and still do) with both genders to prove this.
On the other hand, sexuality is such a complex part of humanity. I think there are all sorts of attractions you can have to the same sex, whether your gay or straight. I also believe that everyone goes through a phase at some point where they question who they are as a person based on who they like. I am one of those people who believe that no one is 100% straight nor 100% gay, however majority lie on either side of the spectrum, with a few spaced out in the middle. I'd like to think your sexuality is determined by who you fall in love with and who you spend the rest of your life with, whether thats with someone of the same sex or someone of the opposite sex, who cares? I identify myself as a gay male but I definitely would not count out being with a woman if I met the right one, I just see that as very unlikely - and maybe its just the age I am not, who knows? But I just want to take things as they come.