Setting gender identity and/or expression aside and limiting my response to sexual orientation ...
In my experience, people who self-identify as bisexual fall into three major categories. Of course, there are myriad variations within and between each. Human sexuality is a wondrously varied thing.
1. Transitional bisexuals. People who accepted a label for themselves publicly but who privately have become dissatisfied with it as describing them accurately. This group especially contains people who find it difficult to ID as gay or lesbian exclusively. Sometimes that's because of religious or social background. Sometimes it is in recognition of a loving relationship, even a marriage, that the person wants to honor.
2. Either/or bisexuals. These people find themselves equally attracted to men or women, but for them it is not so much the body as the entire package, including personality and soulfullness, that is attractive. These bisexuals include a large contingent who are happy living with a single person of no matter what sex.
3. Both/and bisexuals. These people find life a little more difficult because they are attracted to both men and women, and neither one alone can satisfy them. Some engage in serial monogamous relationships, going from men to women as people come into their lives. Others have multiple lovers simultaneously, some in polygamous relationships, some on the sly. Others are part of polyamorous love circles that include both men and women.
As I said, there are multiple variations of these three groups, and many bisexuals fall between them in some way. I think bisexuality is the least understood communities under the LGBT umbrella. They operate under a prejudice held by both straight and gay people that says that you can't have both but that you must choose one or the other attraction.
In the interest of full disclosure, I consider myself a gay man married to a transman.
When you can transform the war and violence in yourself, then you can truly begin to help others find peace. Thich Nhat Hanh