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?

It's his personal life. As long as he didn't coerce anyone or otherwise cause harm, then there's no problem here. Why should corporate care about what he does when he isn't making them money? Why should anyone at all care? If anything, this is solely between him and his wife, and he didn't marry Nvidia.

Anyway, of anything it's the company that's losing out, not him. He's rich enough to relive a few more lives without ever working, and he's at retirement age. Meanwhile Nvidia now has to handle not having his leadership.