It is a common misconception that bisexuality is just a phase. Most bisexual people remain attracted to multiple genders throughout their lives, even if they enter into a long-term monogamous relationship or marriage.
Even if someone who identifies as bisexual is in a monogamous relationship, they are still bisexual. Sexual orientation refers to who a person is attracted to, not necessarily their actual behavior. Bisexual people are just as capable of remaining faithful in a monogamous relationship as people of other sexual orientations.
Though bisexuality is not a phase, some people may find that their sexuality shifts throughout their lives. For example, a bi woman may find that she is more attracted to men but may find herself more attracted to women several years later. It is normal for these feelings to fluctuate or change over time. Your overall orientation generally remains the same over time, however.
If one of your loved ones, whether it is your child or a friend, has recently come out to you as bisexual, it is important that you take their revelation seriously and don't brush it off as simply a phase. They put a lot of trust in you in coming out to you, and likely feel vulnerable. Do some research and try to understand them and be sympathetic and supportive.