Nic (Annette Bening) and Jules (Julianne Moore) are a married lesbian couple living in the Los Angeles area. Nic is an obstetrician, and Jules is a housewife who is starting up a landscape design business. Each has given birth to a child using the same sperm donor.
The younger child Laser (Josh Hutcherson) wants to find his sperm donor father but has to be 18 to do so. He implores his 18-year-old sister, Joni (Mia Wasikowska), to contact the sperm bank which identifies Paul (Mark Ruffalo) as the donor. The three meet. Joni is impressed by his bohemian lifestyle, and Paul becomes enthusiastic about being in their lives. Joni swears her brother to secrecy as she does not want to upset their mothers. However, Jules and Nic find out and invite Paul over to dinner. When Jules reveals she has a landscape business, Paul asks her to transform his back garden. Jules agrees, although Nic does not like the idea.
While working for Paul, Jules likes that he appreciates her work in contrast to Nic who, Jules feels, never supported her career. Jules impulsively kisses Paul one afternoon, and they end up in bed together, beginning an affair.
Jules and the kids start spending more time with Paul. Nic believes Paul undermines her authority over the children by, for example, giving Joni a ride on his motorcycleâwhich Nic has forbiddenâand by suggesting she give Joni more freedom. After a heated argument with Jules, Nic suggests they all have dinner at Paul's house to ease the tension. Nic relaxes and for the first time connects with Paul. However, Nic discovers traces of Jules's hair in Paul's bathroom and bedroom. When they return home, Nic confronts Jules. At first, Jules denies it but then admits to the affair. Nic is devastated, but Jules assures she is not in love with Paul and has not turned straight; she just wanted to be appreciated. Joni and Laser have overheard the arguments and are also upset at Jules. The household becomes tense and Jules is forced to sleep on the couch. Paul thinks he has fallen in love with Jules and suggests she leave Nic, bring the kids, and come live with him. Jules declines, disgusted with Paul's lack of understanding about their relationship. The children are angry at the both of them.
The night before Joni leaves home to go to college, Paul turns up at the house. Nic angrily confronts him, calling him an interloper, and tells him that if he wants a family, he should make one of his own. Rejected, Paul watches Laser from outside the window, trying to get his attention, but Laser ignores him. Later that night, Jules tearfully admits her errors to her family and begs their forgiveness. The next morning, the family takes Joni off to college. While Nic and Jules together hug Joni to say goodbye, they also affectionately touch each other. During the ride home, Laser tells his mothers that they should not break up because they are too old. Jules and Nic giggle, and the film ends with them smiling at each other and holding hands.

Question: Who does Paul end up having an affair with?
Answer: Jules
Alternative answer: Jules