I have dealt with this prejudice by staying quiet on the topic until asked directly, unless I'm prompted by HP in a certain situation to get political.
Most people think I am whatever they are, and I don't bother to correct them unless they are quite close or we are considering becoming closer.
Some of what you describe, I would understand as issues with polyamory/ethical nonmonogamy. I have met people who feel as you do in the polyamory community, and maybe that is where you could look for support? I don't think your age will be a problem there, many popular and valued members of that community are your age and older.
PS-- I've always preferred bisexual men when I was dating men. I seem to be attracted to people who really are monogamous by preference. That doesn't exclude bisexuals-- it is an ugly and pernicious stereotype that bisexual people cannot be happy and completely fulfilled in monogamous relationships.
Monogamous people can best be happy in monogamous relationships!
For me, sex isn't about the specific configurations of my body or my lover's, but how we express ourselves with whatever we've got. I know many people think the bodies are super important... but I'm (sexually) attracted to the person, or not, and the shape of the bits in the pants does not factor even slightly.