What I think is irrelevant. The OP asked about the blackmail justification for resigning and my point was that in the context of the security regulations as the

What I think is irrelevant. The OP asked about the blackmail justification for resigning and my point was that in the context of the security regulations as they exist, having your affair exposed doesn't eliminate the need to resign. The rules as written require you to (1) not have affairs and (2) tell your superiors immediately if you break (1). Now, maybe you think those rules are stupid. Frankly, I think some of them don't make a lot of sense (or at least fail a cost/benefit analysis), but I don't make the rules. Petreus agreed to follow those rules. If he didn't like them, he could have refused the job. Or he could have worked to change them. But he didn't. So he has to live by them. And he failed to do that.