As a monogamous cishetflex guy, I would offer a hug or other platonic physical comforting gesture to you and say "I dunno, people are weird. Don't let us get you down".

To your point about gender being uncountably infinite, I agree with you - particularly because my gender is only defined by my singular relationship to how I perceive myself (and I think everybody comes to that in their own way). There's a certain subset of "everyone" that just can't grok that someone else could have a different understanding of themselves, and a nonzero percentage of them feels like those differences of opinions devalue their existence for one reason or another. It's totally unwarranted and not the case, but it sucks when they act on those feelings to denigrate another person.

I'm sad that you're getting down about it, but I also want to encourage you - you grappling with this is going to strengthen who you are and I'm really excited for that person.