It turns out he was cheating on her with another man.
Although he's likely asexual as opposed to gay. Possibly why Lloyd got involved with an arranged marriage with Milly.Of course, Nina herself is desperately in love with the prince's dead little sister. Implied to be why Prince Schneizel keeps appearing with Nina Einstein in public - as his aide and brother drop very big hints about his true sexuality.Of course, the act just ends up flaring the feelings she's already been growing for Touma and the 'grandson' is actually a spy from the Magic Side who's been sent to potentially kill her (although he is in love with her). In A Certain Magical Index, Touma is forced by Mikoto to play The Beard for her to fend off a potential suitor (the grandson of the Tokiwadai Chairman) that she's not able to fend off her usual way.
