I agree with most of what you're saying, in my opinion, in general it's just the nice thing to do to refer to someone by their preferred pronouns if you know them, and if you don't do it you're (most of the time) just being an asshole. If you do it by accident, no big deal, nobody actually cares unless you double down cause then you're being rude. Also, as you said it is a mental condition, so purposefully disrespecting someone with a mental condition just because you don't believe them I guess(? I might be straw manning so feel free to correct me) is a bit odd no?
This though really bugs me kinda? The "so no I wont go around encouraging them", you don't need to "encourage" them, that's not really what most people are asking for. Really you just need to accept them and respect how they feel about themselves. That's what has been shown in plenty of studies to be the best way of treatment for them, along with gender conforming care.
I also don't really think using the culture argument as a shield is a very good tactic. There are plenty of cultures throughout history that had views that most would now consider incorrect (slavery and racism being easy examples).
Again though, I agree to live and let live, you're not that bad and I have nothing against you. I just deal with this a lot and have trans friends and the such so I have a lot of opinions :3