He's a hired CEO, his main job is to not seem like an idiot. Integrity etc is important but further down the list. Plenty of people have <100% monogamous relationships, humans are lightly evolved monkeys, mostly with surprisingly little drama. But if you're a professional suit, your image is very important. If you cannot be discreet in your personal life, you cannot be trusted to be the face of a company that employees and customers depend on. The board firing these two also reveals a relatively obscure fact. Media will often depict CEOs as all powerful and high agency villains who, for example, destroy the world by making cheap gasoline. But in 99.99% of companies, the CEO is extremely disposable, as we have just been reminded. Their job is demanding and failure is not permitted. Note: It's much easier to avoid being caught having an extra-marital affair by simply not having an affair.

For example, it's not exactly a secret that Feynman's third marriage was open. Or that Oppenheimer seduced many of his colleagues' wives. But it wasn't their job to present to the world as steady handed business leaders, nor did they stake their authority on some kind of appeal to superior morality. Their reaction to being caught on the Jumbotron gave them away - mens rea.

Note this is also somewhat distinct from Founder CEOs, who are rare, particularly ones who both control the board and have a public profile. And whose main job is to create a trillion dollar business.