I heard it argued that the breaking of the story in relation to John Edwards was a public service, as he was hiding a scandal from his supporters and donors that would have devastated his chances of election had he been nominated for the Presidency, and similarly that the disclosure of Anthony Weiner's exploits on Twitter was a public service to the voters of New York who might have supported him in a run for mayor. The implication of that argument is that, absent an election, the behavior of both men is irrelevant to their public lives. I've also heard it argued that if you take Newt Gingrich's infidelity while married to a cancer-stricken wife and combine it with Arnold Schwarzenegger's love child you have John Edwards - but Gingrich is deemed a serious presidential candidate and Schwarzenegger's image is at worst a bit tarnished. Is that fair?
Well... yes. It isn't so much that Edwards suffered a collapse of his public persona because he had an affair that resulted in a child, so much as it is that he was revealed to have a private life completely at odds with the persona he was selling to the public. And when it looked like he was going to be caught, the cover-up was pretty outrageous.
But I think there's something else that the "It's their private lives" crowd is missing. The question of whether you can neatly compartmentalize fidelity - "He cheats on his wife and fakes his religious fervor, but I trust him to be faithful to his oath to uphold and defend the Constitution." Perhaps some compartmentalization is possible. Perhaps Bernie Madoff would never have dreamed of stealing from the collection at his synagogue, even as he happily ripped off his investors.
Perhaps with some relationships the flexibility of the marital vows is part of an explicit understanding between the partners (although that was pretty clearly not the case with Edwards - although that brings us back to the issue of lying to voters and supporters over something that could blow up during an election. Perhaps you can rationalize the deception in those cases - it's society that's being unreasonable in holding that a perfectly qualified individual is "unelectable" if he has an open marriage or is an atheist, and thus he's justified in pretending otherwise in order to obtain and hold office. But it seems to me that most people act pretty consistently across their lives - if they don't steal at work, they don't steal at church. If somebody like Madoff doesn't steal at church it's probably because it's too little money to be bothered with - but if that church were to have a considerable endowment and were to invest with him, "game on".
I personally believe that somebody who is dishonest in relation to the most central personal relationship of their lives is likely to display the same level of dishonesty in other contexts. Liars lie, cheaters cheat, and thieves steal. Some dubious behavior may hint at personality traits that could be beneficial to a political leader - if we're honest, there are times when a psychopath's sang froid, ability to fake emotion, lack of conscience, and ability to lie without the usual outward signs of deception could be useful to a President. That's not to say we want a psychopath in office, but maybe we do want a guy who can have a huge scandal break just before his State of the Union Address, and who will nonetheless proceed to present that speech as if nothing untoward has happened. Both the human psyche and the world are complicated. But if you're electing a thief, liar or cheater, you should have the opportunity to know what you're doing.
Update: In the New Yorker, Hendrik Hertzberg argues that
[Weinergate] confirms a pet theory of mine: the Clinton Rule, which states that when a married politician appears before cameras and microphones and starts babbling absurd lies about some sexual something, the person he is really trying to lie to is his spouse. The lies that get told to the public and the press are side effects.and
By itself, the fact that a person has lied about sex tells you nothing about that person’s general propensity to lie.... If the politician is a habitual or characterological liar, the public record will show it and the lying-about-sex is redundant. If the politician is not a habitual or characterological liar, his lying-about-sex is misleading—is itself a lie, in a way.Hertzberg assumes that the public record will be both available and clear. It may be that it's the lies about infidelity that lead people to look more closely at the rest of a politician's record. But he's also compartmentalizing behaviors, as if you believe that a person's essential character is consistent across the board, somebody who cheats on his spouse is in fact likely to display similar conduct in other contexts. Even if we limit it to "He'll only lie to me if it's necessary to save his political career," and even if I think that a public reaction to his infidelity would be exaggerated and thus sympathize with his desire to keep it secret, I don't see why lying to save his career in this context would not suggest that he will lie to save his career in other contexts. Frankly, if the argument is that the public needs to shift its opinion on infidelity and be less judgmental of politicians' affairs, even if there is an individual price to pay before cultural perceptions shift, politicians would do themselves and their peers a much greater service by being honest.