Why are homos and heteros scared of bisexual adultery?

One of the many classic reasons given by heteros and homos as to why they don't want to date someone who is bisexual is that they would be scared of the bisexual cheating on them or leaving them for someone of the opposite gender to themselves.

And it confuses the shit out of me. Not just the frustrating assumptions behind it that we can't be satisfied with only one gender; that we need both, must have both, and probably at the same time, or we're somehow going to spend our entire lives having threesomes, or at least sleeping with one person of each gender at all times (which to ignorant people means 1 man and 1 woman), or even that we by necessity are all polyamorous, and the only way to be happy is a three person couple.

Not just that. It hurts, like all biphobia, because it makes me feel like I am automatically untrustworthy. Why should who I am attracted to have any bearing on the kind of person I am? I'm a good person! There are cheaters, and there are faithful partners, and neither group is delineated along the lines of orientation. If I'm the kind of person who wants a monogamous, long-term relationship, and eventually marriage, the genders of the celebrities on my personal Hot List are irrelevant to my likelihood of cheating/leaving my partner for someone else.

It also seems illogical. Who cares what gender my partner has cheated on me with/is leaving me for, that bastard has betrayed me, replaced me, broken trust, probably lied to me, and disrespected me! My girlfriend sleeps with/runs off with a man, it is not going to hurt any more or any less than if she had slept with/run off with a woman. Even the thought of this non-existent girlfriend cheating on me hurts a little bit; the image of someone's hand, anyone's hand, touching the body that I - hypothetically - loved, caressed, cared for, and centred my world around - how could it hurt more, simply because it was Craig and not Carol? The girlfriend would be gone, my heart would be broken, and I would hate the person they slept with equally if it was Craig OR Carol.

So why do some homos and heteros feel differently? I genuinely don't understand. Sure, someone of a different gender to you can give your partner different things. But obviously so can someone of the same gender as you, otherwise why do homos and heteros cheat on their partners with people the same gender as their partner? (Hopefully that sentence makes sense). If your partner has cheated on you/is leaving you for someone else, you have been inadequate or something has been wrong; it is not your gender. It's you, or it's them, but it's probably both of you, two individuals with unique lives, with a broken relationship. Don't blame their bloody orientation - you're sidestepping the issue.

And don't be scared of entering a relationship with a bisexual. The relationship will work or will break, based on how the two of you work together, and how much effort you both put into the relationship.