Do Not Disturb: London, The English Countryside, & Paris
It's not too often that you find a romantic comedy (or any kind of movie dealing with relationships) where the two main characters are already married. I was happy to see that the Doris Day/Rod Taylor movie, Do Not Disturb, fit this description.
Too bad it doesn't have a better script. (Day's character is often ditzy and annoying, and Taylor's is boorish.)
But at least it doesn't disappoint visually. The film is set in London, the English countryside, and Paris. And although I'm sure it was mainly shot on a back lot in Hollywood, the sets -- and especially the clothes -- are beautiful.
Can you think of some good movies about two people already married or in a long term relationship (where the relationship is a main focus of the film)? Off the top of my head, the only one I can think of is Julie & Julia (the Paul and Julia Childs part).