Identity has nothing to do with sexual behavior or experience. Bisexuals, despite the sexually insatiable stereotype, may or may not be sexually active, may or may not have been sexual with more than one person, or may never have been sexual at all. As with all sexual identities, whom one is, or is not having sex with, or whether one is being sexual or not, has nothing to do with the validity of a self professed identity (i.e. a lesbian is still a lesbian, a gay man is still a gay man, and a heterosexual remains a heterosexual whether they are being, or have ever been sexual, or not).
Bisexuals are very diverse. It would be a mistake to assume they all fit one or two patterns. (For example, a bisexual married man may have a different history and set of issues than a single woman who has dated both men and women.)
There are many different life histories that may lead to seeing oneself as bisexual. Here are a few variables one might use to classify personal histories:
Bisexuals in long-term relationships can be almost invisible, unless they "come out" deliberately, because their sexuality is judged by their primary relationship.
Some bisexuals say they get much same needs met from both men and woman, others say their experiences of the two sexes are different. Bisexuals are not always equally attracted to both sexes.
It is a myth that all bisexuals must be involved with multiple sexual partners. There is as much range and variation among bisexuals as in any other group. Various bisexuals practice life-time monogamy, serial monogamy, poly-fidelity, open relationships, one-night stands, and celibacy.
Try not to not confuse conventional sex roles with sexual orientation. For example, while some bisexuals are attracted to the idea of androgyny, others are not. (Jungians seem especially subject to this error.)
Some bisexuals get support and identity from the gay or lesbian communities, others feel relatively distant from them. Some bisexuals are found in other sectors of society that are relatively tolerant of diversity (for example, science fiction fans or neo-pagans).
Some recognize signs of their bisexuality at an early age; others realize it only after a pattern of feelings or relationships which takes them by surprise.
While we may be open to partners of either sex, in general bisexuals are likely to be as discriminating about their sexual partners as anyone else.
At the same time, we do not always "choose" the object of our attraction. We are as subject to attacks of romance or lust as anyone else.