Because our culture generally assumes monogamy (and is generally correct), when we settle down with long-term partners people then assume we are straight or gay.
Even Robyn Ochs, national bisexual activist and advocate, was labeled a ‘lesbian’ by media outlets on her wedding day. The reason it matters is that we have terrible health outcomes compared to the straight and gay community and we feel lonely, isolated, and invisible. Seeing us and honoring us for how we identify is one way you can help us to stay happy and healthy.
*a word on the binary: many queer folks of my stripe don’t like the word ‘bisexual’ as it seems to support the idea of the binary. I respect and honor them, and generally use the word ‘queer’ myself as a result. That said, the original reason many bisexuals took on that word was as a way of claiming the psychological description of queers as ‘bisexual,’ as in: having neither or both male and female characteristics, which I see as being trans* from the start, or genderqueer, or at least subverting the binary. This is an issue up for vigorous debate, of course.