explain the purpose of the stevensons

Early in the story, Mike and Frances are talking about how they will spend their Sunday. They consider going back to bed together, and Frances mentions that the Stevensons have asked them to come over around 1 p.m. They will all leave New York for the afternoon for a drive in the country. Mike playfully dismisses the idea, indicating that he'd prefer to spend the day with Frances.
As the day progresses, Frances confronts Mike about his wandering eye, which he initially tries to defend as harmless. However, the conversation about his tendency to check out other women eventually turns serious when he admits to Frances that "I love you, but I also want them."
Frances becomes pleading and tearful, and Mike admits that it is conceivable that one day he will act on his desire for another woman. Frances absorbs this confession coolly and suggests that they go for the drive with the Stevensons after all. It seems that the Stevensons become at least a temporary buffer for the profound tension Mike's admissions have created. It seems that Frances at this point has had enough of being alone with Mike and wants a diversion from the intensity of their conversation.