the immediate thought is how it pertains to themselves---if the man is attracted to women, well, that means they're "safe," they're Obviously not gay, so there's no reason to find homosexuality threatening, right? but bisexuality throws a wingnut into that line of reasoning, and thus, their existence is more fundamentally threatening; it's a reminder that simply being attracted to one gender doesn't necessarily cancel out attraction to the other. it's ninja gayness! it could still get them!