Even Greener Pastures

I was going to add a 2nd edit to my first post, but now it seems to me the object of this endless discussion (about "He's gay" vs. "He's a gay")

https://forum.wordreference.com/threads/he-is-gay-he-is-a-gay.2566761/

may have had some impact on the question of whether "vegetarian" is preferable to "a vegetarian" - at least for some people.

tl;dr
"he's gay" is both correct and idiomatic. "he's a gay", on the other hand, while not being ungrammatical, is a) unidiomatic and b) derogative.

I'd seen the first of the two wordreference discussions prior to writing my post but hadn't seen this one. "A gay" is seen as pejorative. So that does sound like people don't want that label. It's a bit like them saying "it's just one of the things that describe me (adjectives), it's not who I am (a noun).