I dont think they should have fired him if thats the issue. The relationship was consensual what I find wierd is that they expected him to disclouse it sort of no personal life at all.
If there is a company policy in place, I would assume that it prohibits people in positions of authority or influence having "relationships" with people in a position to benefit from that relationship, or potentially being coerced into the relationship.
This is a good policy, and protects all parties involved, as well as the company, itself.
If the CEO of a company can't abide by the rules he either created or, at the very least, signed on to, the company has every right to can him. As it was consensual, I hope they'll be canning her as well.
He is married. If he was divorced, it may not have been as big an issue.