Here's the thing...no one can "prove" their sexual orientation. It's all a matter of who we are attracted to romantically and/or sexually. And the fact is, if someone doesn't want to believe what we say about who we're attracted to, there's nothing we can do to "prove" it.
Anyway, how would someone "prove" that they're bi? Post a hot video of a threesome online for all to see? I guarantee, even that won't prove it to the naysayers. At the end of the day it's just a matter of taking someone's word for it, but that's apparently too much to ask.
I look forward to the day that no one is asked for proof of their sexual orientation, and also when it's no big deal to find out that someone is queer.