Happy Few / Four Lovers (2010)

Rachel (Marina Foïs) works in a jewelry store. When she meets Vincent (Nicolas Duvauchelle) in the workshop, she is seduced by his frankness and decides to organise a dinner with their respective spouses, Franck (Roschdy Zem) and Teri (Élodie Bouchez). The two couples barely have time to become friends before they've fallen in love. Without wanting it, totally spontaneously, the new lovers become inseparable. They walk blindly into their passion, without rules and without lies. They keep it secret from their children and everything continues, almost the same as it was before. But what binds them together is so strong that confusion soon settles in. Feelings entangle and questions become more and more cruel.
Imdb