Jealousy is toxic, regardless of the parties and sexualities involved. But, it's also a natural, animal emotion. The difference here is there is a built in assumption (sometimes justified) that a certain "type" of person is more likely to cheat or run off to "greener pastures."

This particular insecurity often comes from real experience, coupled with the jealous/paranoid person's awareness that they could never provide the physical attributes that attract their bisexual partner. It goes both ways. Especially with bisexual people. A woman knows her bisexual male partner feels attraction for people who have body parts she doesn't have. And, a gay man knows he can't provide tits and vagina for his bisexual boyfriend. Comes with the territory, I guess.