I think what you wrote here is all valid and insightful, but I have always thought it was something similar but simpler: bisexuals get a choice, nobody else does — and people feel threatened by that.

Gay men and lesbians see bisexuality as undermining the notion of “I can’t choose who I’m attracted to/nobody “chooses” to be gay” if people start to see bisexuality as part of a spectrum, well, then maybe their opponents will say everyone has a “little” bit of a choice and maybe you don’t need the “special priviledge” of marriage — thus biphobia is reduced once gay marriage is in hand.

On the other hand, straight people see bisexuals as threatening to the heteronormative narrative: those people *could* just as easily choose the “straight life” but they might not, which means there is something lacking in it that is not innately right or appealing. Gay men/lesbians have no choice but to live an alternate lifestyle, so straight people give them a pass (and a hefty dose of pity), but a bisexual? That is inherently threatening.