Puccini for Beginners (movie, 2006)

When her inability to commit leads to a breakup with her girlfriend, opera-loving writer Allegra winds up in the bed of amiable professor Philip. He is so smitten with Allegra that he dumps his lover, Grace (Justin Kirk), and convinces Allegra to continue their affair. When Allegra meets Grace, sparks fly, and she begins a parallel romance, unaware that her new lover is the woman Philip left to be with her.