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James and Sirius would be using the mirror and when they get interrupted (not sex, they're just talking) by one of James's parents coming into his room, Sirius would fake a loud, shameless and girly moan, which results in James slamming down the mirror so hard it almost cracks, which ends the mirror connection.
Mia/Monty does indeed think he's watching porn and it results in an awkward silence, then a flustered, stuttering James trying to explain himself, which makes him look even more suspicious.
Sirius is losing his shit, cackling on the other side.
...after so many similar incidents, with Sirius pulling the same joke, James eventually learns to end the mirror connection to not give him the opportunity.