In a high magic setting with easy access to polymorphing. The very concept of transgender as we know it basically would not exist.

Your more likely to have prejudices against trans-species people. Since in most d&d settings, gender plays almost a non-factor of a role to most parts of society due to magic and tools being available.

While species tend to be rather insular and there is large amounts of conflict between territories and species.

Dwarves versus elves half elves the draconic races and kobolds for the semi-civilized races such as goblins or knolls.

So having say human that identifies more in line with orcish cultures in the orcish mindset, polymorphing himself into an orc would probably face serious backlash from more conservative or traditional orcs.

Dwarves, for example wouldn’t really care either about gender but for a elf or a goblin to trying to integrate into dwarven society. Not as their own race but polymorphing into a dwarf would be seen as absurd and extremely bad.

You also have to factor in polymorphing in almost all fantasy settings is generally speaking extremely frowned upon if it changes your race or your identity in major ways.

Since it would be a common thing that spies assassins, thieves and other such unpleasant folk would do. Not to mention high-level mimics dragons and other polymorphic creatures that would infiltrate society to eat humans elves dwarves and other quote civilized people.

So you also have to kind of think at what level is polymorphing acceptable? If it’s a full ban then yeah you could go all the way to real world examples of transgender prejudice. If it’s only a slight ban or requires you to declare that you are a polymorphic individual then it’s going to be less so.

If you’re going by strict d&d logic and you need true polymorph to make it a permanent change. Then actually having access to that sort of magic would be so unreasonably hard to obtain for the average person that if someone did come across fully polymorphed then that’s not someone that anyone is going to be prejudice against because they could blast them from the other side of a continent with a whim.

So to find someone polymorphed in that case, you’re so powerful that it’s not a factor into your relationship with other entities or civilization. It’s purely a personal thing.

So much of transgender issues come from the interaction between you and the people around you. Gender roles. Gender norms all of that is built up by the society in rules that you grow up in.

Without that society, you really only have the physical personal body dysmorphia issue. Which could be a great reason to become a Bard or a wizard. So you can learn polymorphic magics to fix what you believe is a physical wrong with yourself. But realistically speaking there would be no prejudice problem till after the polymorphing happened given most d&d settings.

Which is also kind of a neat thing to think about. You have a personal, mental and physical issue. You sought out untold magical powers to fix this problem. Society has no issue with you, by fixing the problem, you create a issue that society would see with you. But in doing so you’re also now so magically powerful that no country could bring issue against you as a wizard who can wield 7th, 8th or 9th level magic is untouchable.