To me demisexual is just a subcategory of a broader term of sexuality. Like hetero-demi-sexual. Stuff like that. I don’t find it necessary, but I understand what people mean when they say it.
PC Culture is fucking annoying to me, and I don’t think there’s any more than two genders. But I’m not gonna bother people about it. And I have no reservations about portraying people with those identities or beliefs, because it makes for more wholesome world building. That’s why in ATM4 Kenny is Pansexual and Wendy is genderfluid.
In general, though, they are rare. They only seem plentiful when you’re online.
It isn’t a fanfic unless Main Character has to tear their gaze away from the strip of skin revealed above Love Interest’s waistband when they casually stretch their arms above their head.