You're right, and both Aboriginal and American Indigenous (many) cultures are testament to that. In some case they weren't simply accepted but seen as gifted, too.
And there are so many more examples of this from cultures all over the globe (Indian Hijra's come to mind).

This is a pearl in the ocean of debate on gender and sexuality. A lot of people can’t fathom the fact non-binary genders exist and are accepted in other cultures because they have been socialised by their own heteronormative culture. It’s understandable why a lot of people can’t make heads or tails about the lgbt community for said reason, but if people get out of their information bubble and read expansively (or even better travel) outside of their worldview, then they will gain better understanding just how complex the world is, and that people of non-binary genders are actually just normal people who deserves respect like everyone else.

A lot of people can’t fathom the fact non-binary genders exist and are accepted in other cultures because they have been socialised by their own heteronormative culture.

True

It’s understandable why a lot of people can’t make heads or tails about the lgbt community for said reason....
... then they will gain better understanding just how complex the world is, and that people of non-binary genders are actually just normal people who deserves respect like everyone else.

False. You don't need to understand a persons' relationship with their gender, or their sexual orientation, to respect them. And you definitely don't need to be well travelled or well read to understand that different kinds of people exist.

You might not mean it to be, but this argument is coddling the bigots and acting like ignorance excuses bigotry, when it doesn't.
And more so, as OP and mine and other replies here have clearly shown, probably most cultures on earth have had knowledge of the gender spectrum for hundreds if not tens of thousands of years (because it's part of nature and easily observable without the social constructs), so really there is no justification for that ignorance in the first place, yet it exists, and instead of trying to explain the ignorance you should be asking why it exists and to whose benefit.

My experience talking to people who can’t fathom the fact that there are more than one gender-- and insist there are only two-- is because that’s how they have been taught by their society. So, to me, that is driven by ignorance because they’re not aware that other cultures accept and even glorify non-binary genders. And what is bigotry though if it is not largely driven by ignorance? Fear of the unknown? It doesn’t always happen to everyone but Mark Twain did say that “traveling is fatal to prejudice, bigotry and narrow-mindedness”. Reading about the lgbt acceptance and treatment in other countries is because someone traveled, observed, studied them and published the studies internationally. My last point sounds facetious but that’s an extra ammunition to undermine the bigoted point that homosexuality supposedly “is not normal” and not universal, when in reality some cultures already accept them and all people in those cultures got on with their lives normally.

When you talk about LGBTQ+ people like objects that need studying and understanding and "acceptance", instead of fellow human beings just like you, or pander to those who think that, as if it's a legitimate position that deserves fair consideration (it isn't, we are people, just like everyone else, and exist literally everywhere and have done for all of human history), you're feeding their ignorance and their bigotry and confirming their opinion to them as a legitimate one instead of explaining why it isn't and never was, no matte how much they "don't understand".

When you talk about LGBTQ+ people like objects that need studying and understanding and “acceptance”, instead of fellow human beings just like you

Ever heard of the the term “anthropology”? “Sociology”? “Gender studies?” Various disciplines that study humanity broadly? The entire academic discipline called “Humanities”? I don’t know why you’re so uppity. You might as well accuse the entire scholarly field for “studying” human lives and aspects. What is life if one doesn’t study it and all its components?

you’re feeding their ignorance and their bigotry and confirming their opinion to them as a legitimate one

No! You completely misunderstand. What I’m saying is use the decades of research on human sexuality and gender against bigotry on lgbt! If someone say there is only two gender-- man and woman-- tell them gender and sex are not the same. Male and female are biological sex (there are actually more than one but explaining this requires an entirely different discussion), whereas gender is an abstract concept in which a person is placed social expectations based on the sex he/she is born with. Tell the bigots there are cultures that recognise more than one genders so their conflation of sex and gender is moot!

Whatever, I'm done here, you clearly don't comprehend how treating LGBTQ+ people as a separate type of human that one needs to study in order to "understand" before we can be treated as fellow humans instead of some curiosity (again - there are literally LGBTQ+ people EVERYWHERE, and there have been throughout ALL of human history. We are not "unusual" to the degree you are pretending we are), is being part of the problem, no matter your intentions.

You are affirming the bigotry.

If you actually want to fight it, I suggest challenging your own biases first, and then trying to listen to others who perhaps have more relevant experience than you do.