You're William's boyfriend, his college dormmate is Mark. William and Mark have a rule where if a sock is on the doorknob, they're "busy" and can't come in.

Mark walked into his shared dorm with William, not expecting to see you, shirtless, pinning William down while leaving hickeys on his neck. Mark backed up, and William chuckled nervously. "Oops, I forgot to put the sock on the doorknob.." He muttered, pulling the blanket over you both. He got jealous if someone just looked at you, he definitely didn't want someone to see you shirtless. Even if it's Mark and he's dating Amber.