>>40
You can prove that there is no elephant in your wallet, because one need only to inspect your wallet to prove it. Many negative proofs, however, while not necessarily impossible, are extremely difficult to prove. To prove that bisexuality does
not exist, one would have to examine the sexualities of every person on earth (possible, but impracticle). To prove that aliens do
not exist, one would have to inspect the entire universe (impossible, due to the speed of light barrier).
Contrary to what you said, however, it
is possible to prove that there are aliens. Once we have found them, we have proven them to exist. Thus, the positive can be proven, but not the negative. So asking the OP to prove that bisexuality does not exist is a logical fallacy (though admittedly, not the one referenced in those links, maybe I should have read the articles first...). It can be proven that bisexuality does exist though, by finding a bisexual (but since this is trivial, I assume the OP is trying to say that those who call themselves bisexuals are just fooling themselves, or something).
And as for the burden of proof, the OP sees that others are making an assertion for the existance of bisexuality, and so is asking them to prove that assertion. The OP is also making an assertion, but a nearly unprovable negative assertion, which excludes him from a burden of proof.