In my biased opinion, I think it's absurd to say you can only feel attraction to one sex...not both.
I am bisexual...whether I always wanted to admit it or not, I have been for as long as I've been alive. I have ALWAYS been interested in both men and women equally...even before I knew there was a such thing as bisexuality. I can't provide any factual evidence. I can't show research statistics that prove my viewpoint...all I can do is attest to my attractions, and how I feel. I don't know why people are bisexual, how it happens, or any of that...all I know is who I'm attracted to. So given the attraction that I feel to both sexes, I have to respectfully disagree with the idea that bisexuality does not exist.
I know this post sounds a bit ranty...this isn't directed at you at all. I just feel strongly about this issue, and I've had people telling me for years, for one reason or another, that there is no possible way I could be bisexual. So if I came across as a bit angry, I really didn't mean it. This just kind of struck a nerve for me, so it's my own issues I need to deal with.