I am literaly doing a psychology essay on this exact issue! Research shows that in general, lesbians, gay men and heterosexuals are less willing to be in a relationship with a bisexual than to have sex with or date one casually. I think it could be that there is a threat of "double the dating pool" as in, there is a more perceived risk for a person to lose their bisexual partner to someone else because there are more dating partners to chose from for someone who is bi. Being bi myself, I think its understandable, but silly to think those things. When I am with a person, I am sexually attracted to only them, and no one else matters. I think many people are just very insecure with themselves.