Transgender people, and it's not sex, it's gender I think. It's complicated. But in the world we live in most trans/non-binary people aren't given the dignity of being accepted as they are. They fight bigotry and ignorance each day, trying to prove that they exist. If it takes actual effort (and a lot of it) to maintain your identity in a sea of rejection, these things start to matter. To you it's "yeah, they used the wrong pronoun for me, whatever". But in their shoes this is a huge deal.
Sure, they can shrug it off, but having to shrug it off all the time, every day, will become exhausting. And it's not a lot to ask from us to acknowledge their existence, without any kind of hand-waving about old times and the difficulty of doing it right.