I've been openly bi for years now, but I can keenly recall my experiences being closeted. Back when I was a teenager in school, I was surrounded by people from conservative Catholic families; not a lot of open-mindedness for anything LGBTQIA, to say the least. Somewhere in my mid teens, it dawned on me that my interest in other boys and my interest in girls were both permanent parts of me. Neither was some temporary confusion or hormonal quirk. Previously, my mindset could be summed up as "Well, I am straight... I think?... maybe... er... huh..."
I realized that being openly interested in guys, gals, and non-binary pals was not going to be warmly received by most folks, so I kept it to myself at first. Still, I couldn't bring myself to willingly associate with bigots at school, so when I found out Person X said something anti-LGBTQIA, it was basically a case of mentally going "Okay, so there's an enemy". I guess some folks started noticing a pattern because some of the more loud-mouthed Hitler Youth decided to come after me when they realized I was staunchly pro-LGBTQIA. You can probably figure out what words I heard on a regular basis, even though I hadn't come out yet.
University was liberating; once I was free to start anew, surrounded by new people in an environment that was much more open-minded than that to which I was accustomed, I decided it was safe to seek out other LGBTQIA people. What I found was a mixed bag. Most of the folks I encountered were exactly what I needed in my life; they were open about their experiences and tolerant of the other parts of the community. However, I also learned first-hand that certain gays and lesbians really, really think the community is "LG & Those Undesirable Letters". It was shocking to encounter hostility and prejudice from people whom I (naively) expected to welcome me warmly. It was an eye-opening experience, but it definitely helped me learn.
In truth, I think the only people I personally *needed* to accept me were my family members. My parents and my sisters all supported me when I came out; I like to think that if I had come out to my grandparents before they passed, they also would have supported me. When it came to friends or romantic interests, I obviously wanted support, but in the back of my mind, I knew that if their opinion of me changed, I could always kick their asses to the curb. And lo, some people didn't take kindly to learning I was bi, and it fucking sucked, but in time, I came to accept that their presence could never have benefited me if they couldn't even handle my bisexuality. I'm not some dog begging to be scratched behind the ears. Still, I knew I wouldn't have been able to handle that if it had been my family rejecting me, but life was merciful and there were no problems on that end.
Fast forward a few years. Nowadays, I'm typically in "give no fucks" mode. If I confirm I'm bi and someone has an issue with that, I'll try to educate them if I read them and think they genuinely would take it to heart, but otherwise, fuck em. Gay or straight, if you try to shit on bisexuality, I'll come after you with a vengeance lol.