Disclaimer: Not mine.
Everyone bets on the reason that Montparnasse hadn't beaten the shit out of Courfeyrac yet. Combeferre puts twenty bucks on mere inconvenient timing (Montparnasse's boss, an irate man called Thernadier, of whom no one knew the occupation, wasn't very consistent, and Montparnasse never could tell ahead of time when he would be called in), Cosette bets ten that he's not doing it as a personal favor to Jehan, and Grantaire bets thirty that he doesn't see him as a real challenge. Months later, when all was said and done, Grantaire would walk away thirty bucks richer.
Montparnasse has never seen Courfeyrac as a serious threat. He's just annoying. Let him write sonnets about Jehan's ass. Let the obnoxious kid serenade his poet when Jehan and Montparnasse are at Starbucks. Jehan really loves Montparnasse, and Montparnasse actually likes Jehan, and they have really great sex, and Montparnasse knows that's enough to make him stay.
Still, it doesn't stop him from holding his blonde boyfriend a bit more securely when Courfeyrac approaches.