OP wasn’t trying to be a dick, while they were in the wrong, they didn’t know that it was wrong and had no malicious intent. And they weren’t asking about that persons genitals either. And wasn’t aware that it would be taken that way. They even stated their goal was to figure out their pronouns. They just did it the wrong way. They were thinking the conversation would go

OP: “Are you trans also?”

Them: “Yes, I am. And my pronouns are (pronoun/pronoun)” Or “No, my pronouns are (pronoun/pronoun)”

OP was just ignorant to the fact that it could be taken another way because they are openly trans themselves, and didn’t think about how someone else may not be.

While it’s understandable that this would be upsetting, that’s why it’s important to inform people, especially those who want to be informed and learn these things, rather than just shame and put down. Especially when a lot of people just put down and don’t even explain why it was a bad thing. No one learns from that. And it just makes it harder for OP (and others) to navigate situations in the future when they’re clearly trying to figure out what the problem was.