A thing I love to rant about is the way authors of SFF novels often get really creative with the details of their worldbuilding–magical systems, zero-gravity plumbing, whatever–but then copy over the cultural prejudices of the world they live in without really noticing. An example of this is classic Trek, when Pike says “I can’t get used to having a woman on the bridge.” Really? Sailors in 2021 don’t bat an eye at a woman on the bridge, but Roddenberry thought sailors in the 22nd century would be as sexist as the 1960s. He just didn’t predict how much we would change, socially, in the amount of time that would pass.
Of course, in fiction, we don’t have to actually predict what the future would be like, or make a magical version of the exact past. We can create the worlds we want to create.
Informed, of course, by human nature. Gay people existing is part of human nature. Trans people existing is, too. So fictional worlds where everyone’s het-married and put into gender boxes based on their genitals tend to ring false to those of us who know reality isn’t like that. We find ourselves wondering, “What happens to the princess if she’s into girls?” or “What happens if someone they think is a boy channels saidar, do they recognize she’s really a girl or do they hunt her down?”
You can write a world with homophobia and transphobia if you want to. Some queer people do, to process what they’ve experienced. (I did once, in The Remnant.) But personally, I really love queer-normative worlds. Worlds where we realize cis-hetero-patriarchy is really kind of a historical blip, not something remotely necessary to any society.
In a queer-normative world, you have to consider the social roles of queer people and how the norms would diverge from what we find familiar. For instance: if you have a monarchy, is heredity determined by blood, and if so, what if the heir is gay? Is there inheritance on the uncle-nephew line perhaps? A custom of adoption as equal to blood? Uterine replicators?
In your world, do people “come out,” or does no one assume anything unless they’re told? Is everyone presumed bi until they mention otherwise? Or, like Pratchett’s dwarves, do they keep everything a complete secret?
Is transition possible, either by science or magic? Plenty of fantasy and science fiction worlds play sex changes for laughs (Xanth, Star Trek). Easy enough to do it in earnest. You don’t even have to explain it. Just mention that X character is a woman, and later have her mention that when she was younger, people thought she was a boy. Or, you can make it a plot point that she’s running out of magic elixir and needs to get more.
You don’t even have to have queer characters to have a queernorm world. Simply omitting structures that are heavily based on gender can open up room for queerness to exist in peace. You won’t leave your readers worrying what would happen to queer characters. But it’s good to have queer characters, if you want your characters to mirror the real world. It doesn’t feel real to have everyone be white, everyone be straight, everyone be well-off, everyone be abled, everyone to be cishet.
I don’t think you have to be queer to have queer characters in a queernorm world. I do think you have to be queer to write a story of queer trauma. Otherwise it feels like you’re torturing characters for fun. Other people might feel differently about who should be writing what. I try not to be dogmatic about it; after all, nobody knows what experiences you relate to besides yourself. But I do think we have come beyond every queer story being a heartwrenching queer story intended to make a straight audience feel compassion. Instead, think of making your queer audience feel included.