It's not about the checkbox, it's about them not wanting to conform to a society where everyone picks their pronouns and should learn each other's too, they don't understand it and see it as a problem being accepted as reality instead of treated, and also don't want to society to become overly complicated because of it.

Essentially the problem is that people with low empathy (who statistically also tend to be bad people) can't and won't understand this foreign concept unless they live through it themselves, which usually won't happen.