Comedy Central’s Key & Peele take on using the lavatory while the seat belt sign is on. (HT: Dylan O.)
Basically, to a 98% approximation, flight attendants don’t actually care if you use the lavatory while the seat belt sign is on. They have to tell you that the seat belt sign is on. They cannot tell you it is ok for you to use the lavatory.
I watch passengers, over and over, asking permission. The flight attendant cannot give you permission. Because what if something bad happened? That’s on them and the airline. But if they advise you that the seat belt sign is on and you go anyway it’s pretty much on you.
Now, of course, you must follow flight attendant instructions. So if they actually tell you to sit down, you’d best do it. At some point it becomes a tradeoff — immediate relief and the legal consequences of failure to follow crew member instructions. But I have never actually seen this.
If you can wait until the seat belt sign is off, that’s better, right? But if you can’t, and sometimes you just have to go, go. Try to avoid doing so right after takeoff, right before landing (as in the first and last 5 minutes!). And once you’re in the lavatory, do your very best to avoid this happening.
Note in advance that some of the language is ‘NSFW’.
A totally different question of course is whether it’s ok for coach passengers to use the first class lavatory. And please always wear shoes into the lavatory.