Asklemmy

I too, am a straight male. It's actually pretty easy.

It might be a bit of a faux pas when addressing someone as he/him when they prefer something else. Simply, if the mistake is made, they'll correct your assumption (I'm sure they get it all the time, it's not that big of a deal), and tell you their pronouns. Then it's your task, socially, to respect their wish to be referred to by their pronouns. It might seem awkward to refer to someone directly as "they" or "them", but it is grammatically correct, it just sounds awkward to our brains because it's so rarely used as a singular direct pronoun.... direct in the way that you're talking to, or in the presence of that person... but it's perfectly fine and preferred by your friend/colleague/acquaintance or whatever.

For yourself, if you're commonly and most comfortably referred to as he/him, then you have two options: 1. ignore it, and people will assume, or 2. put "he/him" in things like your bio/email signature/about me pages and leave it at that. It doesn't require qualification or context, like "my pronouns are" or something like that, just "he/him" alone in your bio is enough to let people know what you are comfortable with.

Personally, I don't do anything, I let people assume, because I'm unbothered if someone refers to me as he/him/she/her/they/them. All pronouns for me are fine. I'm most commonly referred to as he/him because it's the historically "correct" pronouns, but pronouns are more or less irrelevant to me.

And yes, people do, in fact, prefer they/them. I've met a few, and it feels awkward at first to say "they"/"them", but you get used to it.