I have to agree with easy1. If you've got a medical problem, tell the crew and we will do our best to accommodate you. But if we say "sorry, the seatbelt sign is on, you'll have to sit down" and you say "
off I'm going", we pretty much are obliged to consider you a threat to safety, or drunk, or on drugs. If you say "I'm really sorry but I just can't hold it any longer" and give me those eyes
which say "no really, I gotta go now!", I'll even open the toilet door for you because none of us wants to deal with that! If pax treated crew with respect and as people rather than blame us for a delay (most probably an ATC restriction), you'll find we can be really very nice, and if you're charming, you may find yourself upgraded, or even better on someone's staff-travel list!