I'm less sure than Michelle that Senator Ensign's affair with Cynthia Hampton falls neatly on the "private life" side of the ledger--not because he is a public figure, but because he was her boss, and her husband's too. Indeed Doug Hampton claimed, in his peculiar letter to Fox News's Megyn Kelly, that "Senator Ensign’s conduct and relentless pursuit of my wife led to our dismissal in April of 2008."
There's plenty that's murky here, but if Ensign abused his authority to punish either Hampton as a result of the affair, that would represent more than a private peccadillo. This would also be the case if he abused his authority to reward either Hampton--with promotions or unusual severance packages, or by pressuring groups with which he had connections to hire them after they left his employ. (An ethics complaint along some of these lines was filed earlier today.)
It's not clear at the moment that Ensign did any of these things. But it's not clear that he didn't either.
Finally, it's worth noting that Ensign might not be the only one involved with some things to answer for, even if the early insinuations that Doug Hampton engaged in extortion have since been walked back to an allegation of "exorbitant demands for cash and other financial benefits."