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.
But if you want to f$ck around and find out, I'm sure it will make for a fun "How I Ended Up On the No Fly List" story.