Musar על שבת 110:6

Since the commission of even such a "shade of an impropriety" can harm the public image of someone perceived as a צדיק, Jacob had to be punished. Dinah was raped, i.e. became someone's sex partner without benefit of the holy state of matrimony. Her brothers described such relations as only being conducted with a harlot, i.e. that only harlots were sexually violated (34, 31). The episode in which Reuben is described as having slept with Bilhah (35, 22) is also perceived as an indirect result of the flaw in Jacob's piety discovered by the guardian angel of Esau.