Fact of the matter is I don’t think anyone sets out to create a vibrant fantasy world, where combat is performed with magic, and still retain boring sexist conventions from our reality.

so it’s easy to accept that sexism just doesn’t have to exist in this world.

Why not? We have supercomputers that can manifest our thoughts into photos at an instant speed. What’s stopping a magical society from being sexist?

If something bad shouldn’t exist because of some other fantastical element existing in its world, then what’s the point of any story, especially Avatar, a story about resisting imperialism and redemption?

Sexism never comes up as a serious issue through the show

Katara’s bending teacher was an old misogynistic guy who thought it was improper for women to bend lol. Without his change of mind, things would’ve been a lot worse for everyone. And it wasn’t even just his own view, it was the view of the society they lived in. She defied it and saved everyone’s asses multiple times.