Disclaimer: None of the characters are mine.
Cosette and Marius are usually a fun couple to be around, but when they fight, it's a war zone. This time it's when he finds out that she slept with Jehan ("He recited poetry to me, Marius, what was I supposed to do?") and Courfeyrac ("Well, he just happened to be there") before they started dating. Marius claims that she should have told him, Cosette claims that it's not of his business, and everyone else tries to avoid them as much as possible. They finally make up after a few days of not talking to one another, and all is well again, though Marius still gets touchy whenever Jehan greets Cosette like he greets everyone else, with a kiss on the mouth, or when Courfeyrac shoots her a flirtatious wink, and the issue is raised again.