Most bisexuals have attractions to both sexes throughout their lives, not just for a period of time.
Bisexuals can and do form monogamous relationships.
The desire to be with only one person has less to do with sexual orientation than with personal beliefs and values. If given the opportunity, bisexuals will always leave someone of the same sex for someone of the opposite sex. Bisexuals, like everyone else, usually base relationships on feelings, not opportunities. Bisexuals do not necessarily view their samesex relationships as less desirable than an opposite-sex relationship. They choose who they want to be with based on their attractions, not the gender of the person.