Anna tries to defend her boyfriend but Seth points out how she seems to be the boyfriend's picture-perfect, obedient lapdog. See, Seth doesn't care as much about his appearance or what anyone thinks of him, so he calls his own shots and can be as dirty as he wants to be -- especially in the bedroom. He casually points out that Anna could benefit from letting loose and being as dirty as SHE'D like to be as well.
Anna is disgusted and leaves Seth be. But his words stay with her... And when she later returns to pick up the car, it seems she's determined to show Seth a thing or two.