What a weird argument to make. It doesn’t add validity to the assertion that a third gender exists but serves as an attack against those you believe to be old-fashioned or not progressive enough.
Why don’t we teach that the Norse mythology is a true and valid religion? After all, you don’t want the “old farts” to dictate society perspective of progress, do you? What about giving validity to the belief that the world is flat? After all, you don’t want to be among those that “simply can’t imagine a world that changes,” do you?
There isn’t a third gender. There won’t ever be a third gender. If a person wants to identify as a potted plant, then let him do so, unless he is harming others and trying to water and fertilize them or trying to convince society that those who don’t identify as flora are oppressing him.