On a commercial passenger plane it's frowned upon.
On planes in general, many people jump nude for their 100th skydive - the original and best video of this has been scrubbed from youtube, but a quick search shows others.
The last time I walked about nude in the main body of a commercial passenger aircraft (nominally a 30-40 seater) it was returning to Singapore from Vũng Tàu with only four people aboard, pilot and co pilot, myself and another surveyor.
Long story, short version - it doesn't always involve sex and isn't always restricted to toilets.
On planes in general, many people jump nude for their 100th skydive - the original and best video of this has been scrubbed from youtube, but a quick search shows others.
Often screaming is included.