You really do need to re-think something, because the evening dress code applies to all of the entertainment venues as well as to the dining room. Aside from the fact that room service and stateroom television are not my idea of how to spend an evening when there's so much else happening around the ship, all included in the fare, the formal nights usually have official receptions hosted by the Captain, which also are formal occasions. It's a major breach of social etiquette to decline an invitation from the Captain while embarked aboard his vessel.
Basically, you have two reasonable options.
>> 1. Re-think your attitude toward "formal" and "informal" nights (note that "informal" is not the same as "casual" and in fact requires coat and tie for gentlemen and cocktail dresses or the equivalent for ladies) and plan to dress as "suggested" (note that the word "suggested" carries an expectation of compliance in this context), as Donna recommended in her reply.
>> 2. Re-think your choice of cruise line, cancel your booking on Celebrity, and book on a line that does not have formal nights.
Here are some alternatives.
>> Radisson Seven Seas Cruises -- "casual" every evening
>> Oceana Cruises -- "casual" every evening
>> Disney Cruises -- I believe this also is "casual" every evening
>> Princess Cruises -- no "informal" or "semiformal" evenings, and formal attire available for rental aboard ship (reserve in advance) for the two or three formal evenings on a typical cruise of two weeks or less
>> Smaller ("costal") vessels (Abercrombie and Kent, etc.) and sail vessels (Windjammer, etc.) typically have only casual evenings
Your travel agent may have other suggestions, too.
Have a great cruise, whichever option you choose.