Fling (2008) on Netflix

Plot summary

Samantha, a twenty-something fashion designer, and Mason, a published novelist who is just shy of thirty, live together and appear to be a happy couple. Their relationship is open: they take other lovers whenever they want and then tell each other about the encounters. Samantha's sister gets married that spring, and at the wedding Samantha is drawn back to her ex-boyfriend James, a guest she did not expect to see. Mason, meanwhile, spends the evening flirting with Olivia, the eighteen-year-old sister of his best friend Luke. Sneaking out of James's room, Samantha is startled by Mason and tells him about the affair.