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.