Okay, I have to jump in with my 2 cents.
Yes, LF, it is a vacation, after all. BUT - the cruise line is pretty clear about what is appropriate for the formal nights. If you choose not to abide by their suggested attire, then you should, out of respect for fellow passengers, opt for the alternative dining on the formal nights.
I always try to follow the dress codes, whether I am on a cruise, in a fine restaurant, at a resort - I think that is fair to all concerned.