Those people are just confused. Bisexual people are really gay, but just won't admit it. It's a stage they're going through - they eventually will choose. Bisexual people often move through a stage of homosexuality on the way to bisexuality. Bisexual people are basically heterosexuals who are just experimenting.
Most people are at least a little bit bisexual.
Bisexual people are shallow and can't commit to one person or even one sex for a long-term relationship. Bisexual people are more open and secure than either heterosexuals or homosexuals in that they are willing to explore their feelings for anyone, regardless of gender.