Publicly, morality is defined by the college as David Lurie is convicted of sexual harassment. In his mind, since Melanie flirted with him, he is only guilty of a minor indiscretion. In the minds of his colleagues, he took advantage of his position as an upper-class figure to exploit a student. Most people outside David's warped viewpoint agree with the latter analysis; Melanie's almost catatonic reaction to David's "not quite rape" is a very significant indication that she never wanted an ongoing affair, and perhaps never wanted the initial encounter.
Privately, David is shocked to find that his daughter Lucy is keeping her pregnancy from her own rape. In his mind, her attackers should be caught and punished severely, but he has no authority to pursue this option. In Lucy's mind, however, they were simply trying to communicate; they were not trying to hurt her, but to display their own natures in the only way they fully understand. For this reason, she views their actions not as a violent, criminal assault, but an expression of human frailty and fallibility. Her personal morality is at odds with most common interpretations.