I don't think a "true" or "pure" heterosexual exists. I believe everyone has some same sex attraction, no matter what they say or how strong/weak it may be. It's impossible to prove, but I definitely believe this.
As far as the "true" bisexual thing....I don't really get it. I mean what, would they add up how much time they've spent with male vs. females and keep it the same? There are definitely different levels of bisexuality. I, for example, considered myself straight until around the age of 25.
I had always had sexual attractions to men, but I suppressed it. There were at least a couple times in m early 20's where I watched a little gay porn here and there,(and feeling ashamed after because of the whole stigma surrounding it) but that was it. Then, my girlfriend started doing a little ass play with me. Then one day we were taking about fantasies and I was talking about watching her get fucked by another guy.
She said"what if he started ******* you in the ass"? I said I would like that. So once she knew and it didn't bother her(in fact she kinda gets off on it, and she was incredibly supportive with all this.),I said enough is enough. I'm gonna do what I want to do and not feel guilty or ashamed afterwards.
Now all that said, I've never had a boyfriend, that's kinda where I draw the line. purely for the sex with men, with females I have actual dating relationships. I just don't feel the same connection with men that I do with females to make an actual relationship work. Plus, I love cuddlin with my thick,soft lady every night.