My dorm has coed multi-stall bathrooms, and over the course of the year, everyone gets pretty well used to barging around with only a minimal regard for privacy. So I’ll be sitting there trying to poop, or drawing graffiti, or whatever, when some guy comes in a couple stalls down to pee. This is not a big deal.
But no matter how cavalierly the guy walked in, if I get up and flush before he leaves, he always always always waits for me to wash my hands and leave the bathroom before cracking open his stall door. Because God forbid you actually see who you were peeing with, even if you already know who they are.
It’s always the guys who do this, too; maybe it’s an extension of that gender-skewed public restroom code that they like to make cheap jokes with on sitcoms.