Good luck finding movies that portray healthy, long term relationships. Everything is drama and dysfunction or drama and romance. These are a couple of movies that actually portray relationships in a fairly accurate and healthy way. Because they are not romcoms (i.e., romantic comedies) or filled with violence, your kid might not watch them by choice. You may have to make this a family activity to watch and discuss together. Who knows, your kid might end up liking them.
Before Midnight (R)
This movie is the third in a series that follows a couple who fall in love at first sight though without a happily ever after (Before Sunrise), meet again years later and become a couple (Before Sunset) and then, in this movie, are in the throws of life changes and marital stress after 9 years of marriage. This shows that the love at first sight thing takes more than that. The movie ends with them together, hinting at things getting better but not all sweetness and light.
Love and Basketball (PG-13)
Finally a movie with a reasonable portrayal of a teenage relationship across time. This movie follows two teens who are brought together by their love of basketball. It follows them across the 4 quarters (cute) of their relationship into their late 20s. They are friends first, they are competitive with each other (one of the two being a competitive, self-assured girl!) and they struggle with unexpressed feelings for each other as their FRIENDSHIP grows. The movies follows them after they, of course, end up together with some hints about adjustments they had to make in their young adult relationship. Remarkable, rare relationship movie for teens.
Here are several more movies that have some healthy and realistic portrayals of established married adults (Parenthood), young adults who develop a relationship across time after initial dislike (When Harry Met Sally), a group of teenagers and their struggles with forming meaningful ADULT relationships (Circle of Friends) and a guy who is dumped by the perfect girl, like in real life (500 Days of Summer)
You can find all kinds of resources including an extensive list of movies that have a point
by visiting the resource pages over on my website.