I'm late to the party, and I've said this before but it bears repeating here.

A lot of it I think, in addition to what's been said already, has to do with the large amount of gays and lesbians who start off calling themselves bi before they realize they're gay. This solidifies the idea in most people's minds that bisexuality isn't real and that people can only be either heterosexual or homosexual.

There's also porn. Many gay porn videos have 'straight guy/str8' in the title because it's not 'hot' if the guy is bi and genuinely attracted to his gay best friend who gets to suck him off. It's only hot if it's a 'straight' guy being convinced or coerced into it. Nevermind that an actual straight man would never put himself in that situation, let alone allow it to be filmed.

On the female side of things, bisexual women aren't really seen as having genuine attraction to both sexes. We are seen as a gateway to threesomes for creepy straight guys and bored straight couples looking to 'spice things up'. If we actually date a woman seriously, that's when the homophobia comes out and it's not hot anymore, it's 'weird' and 'i didn't know you were a lesbian'.