You can measure what sexually arouses people, but you can't measure sexuality. Anything provided here is not enough to nullify the common acceptance of bisexuality as real.
The same goes for evolution. When there is enough evidence compiled against it, that is when we will have disproved it.
I'm not saying this isn't still highly debatable; it is, but it seems like you're meddling in something that shouldn't concern you. It's their own life; why the fuck do you give a damn? Another analogy: if you don't want gay marriage then don't get one.
I was going to mention Bonobos, but then I'd have to get into the genetic complexity of relation and how this also effects sexual-social-behaviour. And frankly, you guys can do the research yourselves. For some simplified information about genetics in relation to homosexuality and bisexuality, I highly suggest Psychology: Themes and Variations by Weiten and McCann. They simplify LH and many of the studies done on homosexuals, heterosexuals, extreme homophobic-"heterosexuals" and bisecxuals. =]