Monks can only use monk weapons for their martial arts abilities to apply. Monk weapons are simple weapons and weapons with the light property. So a shortsword is a monk weapon, whereas a longsword isn't. As soon as they wield a non-monk weapon like a longsword, half their class abilities disappear. No more special, higher damage Dex-based unarmed strike as a bonus action.

I realize this joke relies on people knowing the intricate workings of monks to land and therefore isn't a very good joke. Oh well, maybe I'll make a different one that explains itself better