If unsure, ask, if you get it wrong, apologize and move on, just as you would with a name or title. Showing that you are trying gets you far. It's nothing more than being willing to treat people with respect.
If you honestly want to learn more about the difference between sex and gender, you should seek out numerous and varied sources, ideally from those that are affected.
One of the other posts realised that it took "he/him" as a synonym for man, so instead of "my man" it's "my he/him"
How do you justify behavior enforcement of another human? Even when that behavior might have a adverse effect on their mental health?
This is a brilliant point that is true not just for gender — do you know how many NAMES there are out there? Millions of em! Keith, Sal, Harry, Keith...I could go on. And people actually want me to remember a specific name for them AND THEN address them by that name? GTFO, that's what I say! Nobody has the time or bandwidth to remember those specifics for EACH person they know. Gimme a break.