Let's go back in time for some context.
The French never understood why President Bill Clinton was impeached for the Monica Lewinsky scandal following disapproval by those "puritanical Americans." Sure, he may have lied about the dalliance under oath, but he was just lying about sex. In French culture, it shouldn't have been the voting public's business as long as he was still competently doing his job. (Not that we ever really know how much more competent a person might be in executing his duties in the absence of the extramarital sex; why is the assumption always that they were giving it their all?)
As disinterested as the French may seem about top-level politicians' sexual dalliances, they're ultimately far less accepting of the results. Former French President Francois Mitterrand managed to hide a second family from the public, even as taxpayer resources were being used to support them. The daughter born to Mitterrand and his mistress out of wedlock is now a 37-year-old author and university professor, Mazarine Pingeot, and she's still being confronted in the French media about having been secretly raised on taxpayer funds.
Similarly, the French were happy to ignore former International Monetary Fund chief Dominique Strauss-Kahn's extramarital sexual hyperactivity for years, until it became impossible to ignore. That happened when a Manhattan hotel maid's accusations caught the attention of legal authorities and the American press. Strauss-Kahn has since returned to opened floodgates in France and is currently facing "aggravated pimping" charges in Lille, in connection with a prostitution ring.
It's not as if the French typically have the option of paying attention to such things before they spin out of control. The French media is prohibited by law from publishing personal or private details about a public figure without the public figure's permission. Even when sex merges with a scandal that happens to be in the public interest, French jurisprudence mandates that anything belonging to the private realm must be carefully parsed out of all media coverage.
So, essentially, the French are cool with whatever public officials want to do sexually -- affairs or otherwise -- because they don't really have a choice, since their media are prohibited from reporting such things. If they eventually do find out about such an incident, there won't be any fallout from it unless the misbehavior was accompanied by gross incompetence and not just a lack of productivity.
And here's where the Petraeus affair becomes a very interesting case for the French. Chatter is divided between those who feel Petraeus should be able to sleep with whomever he wants because he has demonstrated competence in his job, and those who believe the case demonstrates incompetence on Petraeus' part by potentially compromising national security.
While French presidents such as Mitterrand, Jacques Chirac and others have had affairs, their side pieces were compartmentalized well away from matters requiring security clearances. Clearly, your love life has spiraled out of control when you're the CIA director and things have gotten to the point where Mistress No. 1 -- when she isn't giving speeches that may or may not include items from your pillow talk, like the possible existence of a secret CIA prison in Benghazi -- is harassing Potential Mistress No. 2 to the point that Potential Mistress No. 2 asks the FBI to get involved.
When your wife throws a lamp at your head, it's your own personal business. But when the FBI has to step in to untangle your "Love Pentagon" and offers you a sword on which to fall, and you can't imagine anything short of swan-diving on it, then things have reached the point of no return. The French book tour awaits, Mon Général!