Ignorance and isolation can do that even to smart people. If you're stuck in a bubble, it can be hard to even see a reason to step out.

I used to hold to some kind of biological essentalism. It seemed obvious to me that people with male physique should be considered male, people with female physique female and while I saw no problem with wanting to get their phsyique surgically and hormonally altered and was on board with respecting the wishes of people on the way to that, I (privately) believed that people weren't "a real {gender}" until they got that "switch". NBs, of course, were firmly outside that binary view.

Without anyone around to actually challenge that biological premise, I don't really see how I could have broken out of it on my own. I also don't think being smart is a prerequisite for being respectful. It's just that I grew up with certain preconceptions and never had a reason to challenge them until someone close to me came out.

It's the confrontation with the unfamiliar and how you handle it that separates tolerance from bigotry, but dumb people can be respectful just as smart people can be narrow-minded.