Grande école

Paul and Agnes have been going out for quite a while and Agnes is shocked to learn that he'd rather live with two roommates on campus than move in with her. As soon as he meets one of his roommates, Louis-Anault, Paul's behavior changes - he is attracted to Louis without realizing so himself. Agnes, on the other hand, gets quite jealous and offers a bet: Whoever gets to have Louis-Anault first, wins (imdb)