Many do, for a number of reasons. 1) Resentment that bisexuals can simply slip right in with heterosexuals, if they so choose. They can fall in love with a woman, be married, and be normal. They never really have to come out as not straight if they dont feel like it. Just enjoy the perks of gay sex and live a straight life. 2) Some percieve that bisexuals are more likely to be closeted, and less likely to be out and supporting gay rights. They feel the whole gay community must stand together, and bisexuals are more prone to stand back and watch. 3) They feel bisexuals are not trustworthy. They have only known the feeling of being attracted to one sex. Just like heterosexuals who cannot understand gay people, they can't see how one can be interested in both sexes. They fear a bisexual will never be able to commit, never be satisfied. 4) They may be afraid that a bisexual will leave them for the opposite sex. No one likes to be dumped, no one likes to see their ex with another. But with the animosity between the gay and straight communities, they feel the bisexual is a traitor to his own kind. They feel betrayed. It's not fair, but its human nature to be resentful, jealous, untrusting, and fearful. I dont blame them. But I wish they'd see that as a hated minority, they should do their best to try to see the other perspective. At the end of the day, they're both hated sexual minorities, and have more in common than not.