Why does Petraeus have to go?

I suspect that someone in the military with an axe to grind ratted out David Petraeus for having an extramarital affair. I am told that President Obama tried to talk Petraeus out of resigning, but Petraeus took the samurai route and insisted that he had done a dishonorable thing and now had to try to balance it by doing the honorable thing and stepping down as CIA director.

But why? Petraeus is retired from the military. If the affair happened back when he was on active duty, it is part of the past. And there is nothing illegal about civilians having affairs.

So the surprise to me is that Obama let him go. But the administration’s loss may be Princeton’s gain.