I think homophobia is discrimination based on same sex attraction. There are (at least theoretical) cases where someone would discriminate against homosexual and not bisexuals, but for the most part, homophobia is directed at all people with same sex attraction, regardless of whether that same sex attraction is exclusive or not. Bisexuals may be better suited to hide from homophobia like that by only acting on their attraction to the opposite sex, but avoiding discrimination by hiding facts isn't the same as not being the target of such discrimination.

To be honest, homophobes are too stupid to tell the difference. We're all just f@gs in their eyes, regardless of whether we're gay, bi, lesbian, or any other quality of who specifically we're attracted to and what kind of relationship we want. It may be argued that such people would simply be both homophobic and biphobic, but since the bigotry is based on the presence of same sex attraction, I think it should be all one thing: discrimination against (exclusive or non) same sex attracted people