It’s Usually OK to Use the Toilet Even When the Seatbelt Sign is On

The flight attendant can tell you the seatbelt sign is on, but that’s OK.

http://viewfromthewing.boardingarea.com/2015/07/17/its-usually-ok-to-use-the-lavatory-while-the-seat-belt-sign-is-on/

However, if s/he says that you must return to your seat, you have to obey.

The airline really, really does not want you to soak your seat cushion in urine…it’s so darn hard to sell that seat to another passenger on the next flight.