When Roxane asks what it means to be "mono gamous," she's referring to the fact that you can love more than one person. Roxane says that she and Caius have been "flirting since junior high" and that Caius is "still madly in love" with her. They decided to let Caius join them and set some boundaries, and promised to always commu nnaicate and listen. Caius and Roxane break up because Caius wants to "move away and think it work" .