I think the issue is that a lot of those headcanons are (ironically) based off really token stereotypes. It’s the same reason that I don’t like autistic headcanons (despite the fact that I’m on the spectrum myself), it comes across as condescending instead of truly progressive/inclusive. I’ve also met people who’ve used autism as an excuse for their shitty behavior, so I don’t want to associate characters I like with said people.

There’s nothing wrong with projecting a gender or sexual orientation on a character, but it’s not cool to call others bigots for not agreeing with your interpretation.