It isn't so much the morality of it. CEOs as a group aren't known for excessive scruples. It's the piss-poor judgment he exhibited. Even if everything was consensual, this is a blatant violation of corporate standards and inevitably diminishes his reputation and therefore his effectiveness at his job. He's not only borked his own career, he's lost a ton of money for everyone connected with Nvidia. The question really is, why was he so stupid to think that he wouldn't get caught or that it wouldn't matter?