Bi-umbrella is vast: we have pansexuals, bisexuals, polysexuals, and more. As you are probably aware, bi+ are part of the LGBTQ+ community and a high percentage of the community. But did you know we are the one less taken seriously? Bi-erasure is a thing and it needs to stop.
First, what is bi-erasure? Bi-erasure is the action, the tendency of ignoring, falsifying, or the belief that bisexuals don't exist. There are several kinds of bi-erasure. For instance, accepting bisexuality in women but not in men. Bi-erasure is caused by misinformation, stereotypes, and lack of representation. Straight and gay people assume that it is a phase. For some, coming out as bisexual is a stepping stone for their next step, mostly done for a safe route. Doing that, however, does not apply for everyone under the bi-umbrella. Dating someone of your same gender or opposite gender doesn't change your sexual orientation. If you identify as bisexual you will remain bisexual if you are dating a man or a woman.
Second, what can you do to stop bi-erasure? If you encounter a bisexual person, do not say that is not a thing, do not say you have to pick one side. Here is the thing, yes we are attracted to two or more genders but that does not mean we are greedy. In fact, bi-erasure started due to the belief that we sleep around and cannot have a monogamous relationship. The exclusion of anyone under the bi-umbrella started within the LGBT+, mainly because we did not fit anywhere. After all, it is one or the other one but not both.
The exclusion has taken the form of omitting the bi word in the name of organizations or events that are of the queer community. If kept, they would treat bisexuality in a derogatory way. Back to the point, you can start by acknowledging that everyone under the bisexual umbrella exists, acknowledging their sexuality. Do not force them into any situation they are not comfortable with, for example, threesomes. We have dealt with several sexual cases of abuse based on our sexuality due to misconception and erroneous beliefs.
Third, do your research, ask questions. It doesn't harm anyone for wanting answers. You can read about this topic, go to forums, and go to events. Through your experience of seeking knowledge of the LGBT+ community to keep an open mind, try to push your beliefs for a new point of views.