Let me put it this way.
Say you were filling out a form for something that didn’t require any medical or physical attention, say it was a form to apply for a free pair of shoes. Lets say for the sake of this argument you identify as male. When you get to the part where it asks your gender (gender, not sex), it only has the option of female. You might stop and think something like, “Wait, but that’s not me! How come it only has an option of certain people? That isn’t fair to me and what I identify as, and it doesn’t give an accurate representation of who I am.”
For non-binary and some trans* people it’s like that for everything, even something as little as signing up for a website. They’re caught at having to choose a gender they are not, and I can say first hand it makes a person feel bad because they are being asked to choose a gender to represent them that doesn’t represent them at all.
It’s even worse when the option is specifically labeled “gender” and not “sex”, two thing that are separate from each other. I’m a little more put at easy if the category is labeled Sex and had the options Male and Female, but if it’s labeled Gender and only has Male and Female, that’s incorrect. Gender is not just male and female. A simple “other” option completely suffices, since there any many other genders and listing them all would take a LOT of time, and male and female are still the genders that the majority of the world’s population identify as. But it gives the people who are not male or female an option, and honestly makes them feel better about filling out that section.
Now the whole thing with pokemon is ridiculous in my opinion. Would it be cool to have an androgynous character to play as in pokemon? Hell yeah it would! But what would they look like, what would they wear? And when it comes down to it, pokemon is a role playing game, the character is not meant to represent you. If it were meant to represent you, you would be able to customize your character.
When it really all boils down, being asked to give your biological sex when you’re a person who tries to hide it in one way or another, whether that person is trans*, bigender, agender, etc., it’s pretty demeaning. For instance, say you were bald, but you do not feel correct being bald, so you wear wigs and do your best to make it look like you have hair. You feel confident with your wigs and you like yourself with them, and other people you know don’t know you are bald and you like that, because you like yourself with hair. But when you’re given a form and it asks whether you are bald or if you have hair and you know you should check of the bald because thats whats true, you are in fact biologically bald, but it’s just not you.
Reading that over again that might not be a very good comparison, but I hope you get what I mean. I hope this helps!