Tobi and I watched a documentary on TV this week called Inlaws and Outlaws. It's a interwoven set of interviews with couples who have been together for a long time. Most of the couples are gay.
I liked the film because it did not sugarcoat the challenges of having a successful long-term relationship. Tobi and I paused the movie a number of times and discussed some of the issues it brought up. It was very thought-provoking. It was inspiring as well, since many of the couples had built very strong and loving relationships.
A trailer can be found here. I can really recommend this film.