Here's a thought that might help us to answer Fred Kaplan's question "Why?":
How can he, Bill Clinton, endanger his presidency so? Of course, men take weird chances when the navigator at the center of oneself whispers in the dream: Kid, your cancer is near.
For some, the cure for cancer is to visit the moon of moral peril. If the cause of cancer is undissolved shame, and cancer is a revolt of the cells against the hegemony of the CEO (that mysterious Chief Ego Officer who runs the body), then it may be that Clinton is full of undissolved shame. Let us warrant that it is not because of oral sex.
His shame, if he has any, is that he has never been able to stand up to the big money. He is powerless before men of huge financial size. Face to face with such buckos, the wind dies and the proud flag on the flagship commences to droop. As Monica Lewinsky is to Bill Clinton, so is Clinton to the big money--just a kid trying to earn his presidential knee-pads.
If it all comes to the worst for him and he is obliged to resign, a denouement which seems unlikely at this writing, well, an old moral law will have been observed: The criminal is rarely condemned for his true crime.
Nixon's sins in Watergate were venial compared to the monstrosity of allowing the war in Vietnam to wind down over four years while two million more Asian men and women were killed. Clinton's major crime is not that he has charged relations of one sort or another in the White House (that palace of presidential purity!) with a young girl, but that he betrayed the poor and enriched the wealthy. (Norman Mailer, August 2, 1998, excerpt at Amazon.com)
I think it is important to keep in mind that Petraeus' "sin" here is decidedly venial when compared to what his agency is involved in in, say, Pakistan.