The best "adults only" dining in the Epcot area is at the Swan and Dolphin. Resorts They are a mere 10 minute boat ride away, or a walk of about equal duration. You can re-enter the park in plenty of time for Illuminations. If dining there, I would suggest bluezoo, Il Mulino and Shula's, in that order, unless you greatly prefer steak to Italian fare, in which case I would reverse Il Mulino and Shula's. The food at these three restaurants is orders of magnitude better than anything at Epcot other than Mssr. Paul.
If you intend to remain in the park and don't want to venture over to the Swan/Dolphin, then I would recommend Mssr. Paul as the best "adult" option, but be prepared to give CPR to your wallet. Le Cellier is a decent choice, but there is nothing particulary "adult" about it. You can take the kids there any time and feel right at home. And really, that same thing can be said about any other reamining Epcot restaurant. Nowhere are you going to find a distinct "adult" atmosphere unless you dine very late...at which point you will miss Illuminations. San Angel is not bad, but it is very dark and the tables are very, very close together. If you are looking for any sort of romantic atmosphere, you can only achieve that here if you don't mind sharing your romance with the three tables that surround you. You may also want to consider Tutto Italia. It is perfectly fine, but I have a hard time getting enthused about it given that Il Mulino is 10 minutes away by leisurely boat voyage. And as with the other Epcot choices, it won't be a particularly adult atmosphere until very late.
