To quote from a post of mine from a previous thread:
"...Now, there's also another theory (I say theory, because I've yet to see proof, but it does hold water) And that's that we're all actually pansexual to start with, yep, that's right, you're attracted to everyone, regardless of gender, or sex, BUT the attraction may be SO small, it never comes up. Let's take our hypothetical forum-goer from earlier, he identifies as being straight, alright, with this theory, he actually does like men, women, intersexed people, transsexual people, the whole nine-yards of humanity. BUT the attraction to every one but females is SO little, that it may never, ever come up (Ie, he may never become attracted to anyone other than women) But, maybe he's got a man-crush on Jason Statham (Yeah, I watched Death Race last night, so shoot me) He may find something particularly attractive about Mr Statham there that it actually overpowers the lack of interest in men, and appear as like I said, a "man-crush" You actually can see this all the time on Youtube, but I'm willing to bet most of those "I'd go gay/lesbian for him/her" Are just for attention, some are probably real.
Also, another thing to note, is that sexual orientation is NOT trinary, it's not just hetero/bi/homosexual, it's a continuum, a sliding scale if you wish"
So yeah, by that logic, bisexual does, and doesn't exist in a sense, it all depends on your terminology. Heck, if we're all pansexual, that kinda makes hetereo and homosexual non-existant, but we use those terms as labels to communicate where we are on that Sexuality Continuum (TM)
Also... bisexuality is SO 1990... pansexuality is totally in (And, to be honest, most bisexual people I've met are actually pansexual, they've just never heard of the term)
And for those umpteen bajjilion people who will undoubtably ask "What the heck is/are pansexuality/pansexuals" http://en.wikipedia.org/wiki/Pansexuality