"What We Talk About When We Talk About Love" features two married couples having a conversation about love. Nick and Laura are newlyweds and have been together about a year and a half. Mel and Terri have been together five years and they've been married for four. The two couples' married lives are compared and contrasted subtly. Laura and Nick's marriage is shown as easy, joyful and uncomplicated. Mel and Terri's is loving, but also a lot messier. But whether happy or unhappy, no two marriages look alike, especially in a Raymond Carver story.
In this story, love and marriage have nothing to do with each other. It's clear that Mel never loved Marjorie, but he married her anyway.
The lesson of "What We Talk About When We Talk About Love" is that without good communication, a marriage won't last. Which is why Terri and Mel's marriage won't last.