Uh... How does that work?
He's having intercourse with the lady past the curfew, meaning that all students should be in their rooms at that time. And wouldn't he most likely go back to his room after the act?
How does his roommate not connect the dot? Was she sleeping at the time? Wouldn't that wake her up and raise suspicion when he's (the "lecherous" guy) pushing past the curfew and therefore missing every night at the same time period such a rumor came to be?
I get the idea of having that funny rumor going around for the author, especially as a way to close the chapter, but it doesn't really work in the way it was delivered...