Well, I think the previous commenters already did.

But if you're looking for the in-a-nutshell version, here's one word that sums it up: monosexism.

That's the basis for all of the anti-bisexual attitudes which others have mentioned, I think. The assumption that sexual orientation = monosexual (because, for most people, this is indeed the case). And that, therefore, there is no such thing as bisexuality: it's not a sexual orientation at all. Hence the alternate "explanations" for it: being confused, indecisive, wishy-washy, immature, slutty, phony, etc. And the erasure (cuz, after all, we don't exist).