Defining bi-sexuality is difficult when you find there are other labels out there such as pan sexual, hetero-flexible, and homo-flexible. Pan, of course is the most confusing to separate out from bi-sexual. I guess a bi-sexual would be limited to only the two human genders and a pan-sexual would bop anything that has human level intelligence? Of course that could leave out some humans.
The flexibles, I get at least. It says that for the most part you are that orientation, hetero or homo, but will not dismiss out of hand a relationship (be it intimate and emotional or strictly physical) with someone of the opposite gender from your main orientation if the conditions are right. With that said, I cite Logan's Law #2: "There is an exception to every rule....including this one." As one who believes that love can be truly blind to things such as gender, especially if you believe in things like soul bonds and life bonds, then a person can have a single exception to their orientation and not be bi, pan, or flexible.
And as always when it comes up about bi's or switches being confused, I pull out this little nugget: Bi, Poly, Switch: I'm not indecisive, I'm greedy.