Yes, there are a few people who think they are bisexual but are really not. But for the most part, if someone has bothered to tell you that they are bisexual that they probably are. This myth bothers me. Bisexual means that you can fall in love with either sex. It doesn't mean that you have to fall in love with someone from each sex at the same time.
Bisexuals can be perfectly happy in a monogynous relationship.
Bisexuals are promiscuous. I, for one, am not promiscuous. And I know lots of other bisexuals who are not promiscuous. Some bi people may be promiscuous, but so are some gay and straight people. Promiscuity depends on the individual.