I think it's been said that having an affair is expressly prohibited in his line of work (given the factors laid out by others here). He broke a really big rul

I think it's been said that having an affair is expressly prohibited in his line of work (given the factors laid out by others here). He broke a really big rule. When you're in such a high position, you don't get to break big rules and then get off scott-free; rather, you will either be dismissed in a dishonorable fashion, or you can resign and save a little bit of that honor. If he had tried to ride it out, it would've created an even greater distraction in the news, drawing even more unwanted attention onto his office and that of his superior, and superiors don't like negative attention. After all of that, he _still_ would have been let go. Get out early and save face.