Even if the student is of legal age when the relationship started, it's still inappropriate in the same way a supervisor/supervisee or other similar relationship is. Sure, it's legal, but there's a certain level of power/domination. Like a professor sleeping with a college student or boss with his up and coming intern.
Minors? That's an easy case, that's right out.
But don't forget the other relationship dynamics as well that can still flag something as inappropriate.