They gang up and track him down

Ron tries not to listen to the girls’ rowdy conversations a few floors below him and just go to sleep, but sitting alone in his room and hearing the echoes of so many familiar female voices - especially the Veela - gets him hard. He lies in bed, frustratedly ignoring the dock tenting his pants, and resolves to fall asleep without touching himself to the voices of his friends.

After an hour or two, unfortunately, the girls fall into a hushed whisper. The silence is closely followed by childish giggling and the creaking of footsteps or the stairwell. Ron bolts upright in bed, panicking. They’re coming upstairs, and he’s sitting on his bed with nothing but a pair of boxers to hide his erect penis!

He looks at his door, noting with relief that it’s locked, but that relief is shattered by the sound of Hermione whispering on the other side, “Alohamora!” The door bursts open, and Ron yelps and scrambles to hide himself as the quartet of ladies barges into his room.