Petraeus resigns over affair

Few details but just now, CIA head David Petraeus resigned because of an extra-martial affair. All the smart people are saying he had to do it because, blackmail. Not sure how that works once the affair has been admitted. What leverage does the rejected lover have once it's made public?

Always being the outlier, I'm not a fan of his at all, but I find it hard to believe he would compromise national security for sex. Think it's more likely he had a fling with a younger woman, because that's what men so often do at that stage of marriage, and wants to spare his wife and family the media circus which would be relentless if he held on to the job. This way, since it appears he's leaving immediately, the media will lose interest in a couple of days once they tease out what few prurient details they can. 

[cross posted at The Reaction]