So, the gossiper was right after all? I mean, the invitation was "want to prove I'm a slut?" and she followed through and proved it was true.
It also proves it's a case of the pot calling the kettle black, but setting that aside, as a test of the accusation the gossiper technically won.
@anon763
But that is the part of the joke. Morishima wanted bully to be more accurate in her accusations. And well, sleeping with her is one way to prove a point.