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.

I don't think we know enough about exactly what happened to determine if he broke the rules or not. People around him in Washington may have known about this for a long time before it went public.

But he's publicly admitted to having an affair. So we know he had an affair. Because he told us.

Hard to know what a person might do in that situation, hence the regulations.

For a lot of these military and intelligence positions, it is best to err in the direction of caution.

Also, the people involved have probably learned from painful experience that traditional institutional rules are usually better guides than they sometimes appear to be at the moment of decision.