"I'm not a hallucination"

To start, Neil ending up in a relationship does not invalidate his sexuality. I don't even know why this is being argued other than the fact that people think he's "not asexual enough" or whatever argument they may come up with that tries to divorce demisexuality from asexuality. Demisexuality is on the asexual spectrum. Some demisexuals never find a relationship, some demisexuals continue to not have sex even in relationships, but many demisexuals do have relationships in which they have sex (just like how many demiromantics have relationships in which they are in love). The fact that they can trust someone enough to have sex and fall in love is kind of the point of demisexuality and demiromanticism; it's the part of the asexual and aromantic spectrums that allows for exceptions like that.

When characters around him don't believe him when he says he doesn't swing, this is an accurate portrayal of how many aspec individuals are treated, both before and after coming out. Asexuality and aromanticism are not respected by everyone (hell, just look within the queer community and you'll even see it here. We're a minority for a reason and denying we're queer only hammers in WHY we're queer. If you believe we aren't, fuck off) and it's incredibly common for people to assume that someone is just closeted gay when they say they aren't interested in sex or romance. The Foxes assume Neil is gay because they don't know about asexuality or any part of its spectrum of identities. The books take place in 2006 and 2007. Asexuality as a term has been around for decades (first recorded in the 1970s, I believe? I could be remembering wrong), but it wasn't very well known until around the 2010s. You really only knew of it if you were in the queer community finding like-minded people, and even within the queer community, those who aren't on the asexual spectrum are less likely to care even now.

Nicky and Matt assume Neil is gay because they don't know about asexuality and assume a guy not talking about girls must be gay. But Neil ending up in a relationship with a man is NOT him "proving them right." He says in The King's Men "I've said all year I don't swing and I meant it. Kissing you doesn't make me look at any of them differently. The only one I'm interested in is you." This is a blatant example of demisexuality. It's textbook representation. The others assuming he's gay does NOT change how he views his sexuality. Why? Because other people don't determine your sexuality. I shouldn't have to explain that to people. Neil knows how his sexuality works and he's confident in that. He doesn't have the specific words for it, but he doesn't need those to know himself.

Also, Neil is not the type to want a label. He will correct people who get it wrong, but he won't care enough to use any word for it. Not labeling it does not make it any less true. He and Andrew don't label their relationship, but the books still end with Andrew admitting they're not nothing. They have a relationship, regardless of the fact they don't label it. Neil's demisexuality is the same way. A lack of a label does not negate the existence of something. I don't have a diagnosis for my sleeping issues, that doesn't mean I don't still have them. A label doesn't suddenly make it real, it just describes your lived experiences.

The need to have every sexuality spelled out and perfectly accepted in stories is a new trend I've noticed all over the place, both in and out of fandom, and I honestly don't know why people expect this. Coming out is messy, having your sexuality respected is often an uphill battle, and often you're not respected or listened to. People are going to push back or not believe you. People are going to get things wrong and you often won't have the right words to explain it. This is normal. This is how it works in reality.

Neil, despite not having the words for it nor caring enough to use them, is incredibly confident in his demisexuality. He asserts that he doesn't swing consistently and without hesitation. Even when he's with Andrew, he maintains it. He doesn't doubt himself at all. He knows how his attraction works and he doesn't question it or change how he talks about it. This is awesome to see because with most sexualities, asexual spectrum or not, you often see characters grappling with their sexualities trying to figure it out. Neil doesn't do that. Once he falls for Andrew, he's all in, but he does not budge on the fact he doesn't swing for anyone else. This confidence is inspiring.