Nic (Annette Bening) and Jules (Julianne Moore) are a married lesbian couple living in California. They have each given birth to a child using the same sperm donor. Nic, an obstetrician, is the primary breadwinner and the stricter parent, while Jules, a housewife who is starting up a landscape design business, is more laid back.
The younger child Laser (Josh Hutcherson) wants to find his sperm donor but has to be 18 to do so. He begs his 18-year-old sister Joni (Mia Wasikowska) to contact the sperm bank and determines that Paul (Mark Ruffalo) is the donor. The three meet. Joni is impressed by his bohemian lifestyle, and Paul is enthusiastic about being in their lives. Joni swears Laser 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 landscape 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, for example, by giving Joni a ride on his motorcycle, which Nic has forbidden, and 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 makes a connection with Paul over their mutual love for Joni Mitchell. 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 insists she is not in love with Paul and has not turned straight; she just wanted to be appreciated. The household becomes very tense with Jules sleeping on the couch. The children are angry at Jules and Paul. Paul thinks he has fallen for 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 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 the kids from outside through the window, trying to get their attention, but they, too, turn away. 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.