No formal nights are not 'compulsary' You can eat in the Windjammer buffet area, you can be smart-casual and eat in the speciality restaurants, you can eat in Johnny Rockets burger bar, you can eat in Sorrentos pizzeria on the Royal Promenade, or you can not get dressed at all and have room service.
Nothing is compulsary! You don't need to dress up on formal nights, although personally I think it's a shame that you don't feel like it as it is the only holiday where formal dining takes place on a regular basis. Even on formal nights, you will see many that are not dressed formal, but who have just stepped up a little, say with a long-sleeved shirt & tie (if you are a man, of course!) or a sports jacket. I don't know the layout of the ship, but it will have other restaurants as well as the breakfast buffet area, parts of which on other ships are converted at night to an informal dinner sitting.
Heather? Aplmac? Where are you?
Thank you all for your answers much appreciated :-.