While Le Cellier is a great restaurant, it tends to be a bit noisy due to the large number of families. For a couples dinner, I would prefer Chef's de France (although it is a bit noisy). There is a more exclusive (and expensive) restaurant upstairs called Bistro de France. San Angel is a very pretty setting, but small and a bit crowded.
While they are outside of Epcot, there are a number of other very nice places for an adults only dinner.
Just outside of Epcot there is Flying Fish and Yachtsman Steakhouse.
At Contemporary is California Grill
At Wilderness Lodge is Artist Point
At Grand Floridian, Narcoossee's or Citrico's
(or for a real special evening, the best Central Florida has to offer, Victoria and Albert's) A dinner event you will never forget.