I know that feel Jul! I met a cool dude recently, and I saw his binder. We were at a con, and he was cosplaying-- so I thought he was just another crossplayer, and called him a she-- He's so good mannered and never corrected me (and a lot of his friends called him she!), but once I found he was trans. I felt like such an ass. I'm making it a point to personally apologize to him next time I see him, turns out he lives near me.