There was a multi episode story arc about a teenager (forgot her name, had a teenage boyfriend and got adopted by Stamets/Cullen) coming out as non binary and choosing they as their pronoun.

This never happened. The only time it ever comes up was in a single 27 second conversation with Stamets in S3 which someone else linked. It was never brought up before or after this scene. This is literally the one and only time any LGBTQ issue have ever been discussed in all four seasons of DIS. I'm utterly baffled how this scene got twisted into some lengthy after school special in at least two people's minds in this thread.

The scene that actually happened wasn't even heavy handed. Adira basically states, "I prefer to be called they. I never felt like a she." Stamets literally responds with just "okay" at which point they move on and never bring it up again. I think you need to question why your mind warped it in such an exaggerated, critical way.