It could be a legal thing. Like they have to use what's on your birth certificate. I've heard of that.

Still, it's rude. I've worked jobs where they've told me to use pronouns assigned at birth. I ignored them, come whatever may. Decency and kindness come first. Respect follows if it's matched, but even without respect, kindness costs nothing. You don't have to like someone to be decent. You just have to be a decent person.

Technically a trans man/transmasc person will always have the XX genes, and there are a few other "technically" statements as well, but it's also quite rude to reduce a person to primary and secondary sex characteristics, especially family. I know people AFAB have girl parts, but if they're in my family, that's not something I want to think about. At all. So that's a bit weird.

I could talk about what makes a guy a guy, but honestly, it differs from person to person. Once we start making rules like that, it actually excludes a lot of AMABs, but we don't really talk about that. I think it's more impactful on the AFAB side. Like women being defined by motherhood, or sexuality related things. It's just a mess, and IMO a bit of decency gets over a bunch of those pitfalls. So like, it's weird, in a sense, to say that a guy is still a guy even if he has "boobs and a vagina," but by the same logic, if we say that defines a woman, we exclude people who have had a mastectomy. My grandmother did — she beat breast cancer. One hell of a lady. Killed by a drunk driver coming up on 20 years ago. So ironic. But no one would say she was less of a woman because she had to have her breasts removed. So I will not go down that line of thinking.

And dude... we're all a bit confused. That doesn't mean you're wrong.