That's cogently argued. But note that it's a cogently argued brief for the view that cultural conservatives ought to deploy the marital indiscretions of liberal politicians as a political issue while ignoring the indiscretions of conservative politicians. Just note that what looks like hypocrisy from the outside can often have a perfectly coherent explanation to the believers.
I can see how it reads that way, but it wasn't exactly what I meant. If the hypocrisy Matt's describing actually worked, in the sense of moving us closer to a standard in which jilting your wife disqualified you from a political career, I suppose there might be something to be said for it. But it's pretty clear from the whole impeachment debacle that it doesn't: All conservatives did in that case was open themselves up to, well, charges of hypocrisy, leading to a denouement in which one of their own, Bob Livingston, fell on his sword while Bill Clinton got away scot-free. So no, I don't think that "cultural conservatives ought to deploy the marital indiscretions of liberal politicians as a political issue." I think that cultural conservatives ought to criticize public figures of both parties when they behave egregiously in their personal lives, and I think that - all things being equal - they ought to reward politicians who don't abandon their spouses, or dally with interns in the Oval Office, or maintain second families on the side. But all things aren't equal, politics is the art of the possible, the Nelson Rockefeller standard isn't coming back anytime soon - and so again, I don't think that McCain's deplorable conduct in the 1970s justifies voting for Obama.
I should add, just in case it isn't clear, that the reason to have a taboo against electing politicians who have ditched their wives isn't because leaving your wife makes you "morally unfit" for the Oval Office in some absolute sense; I don't think it does. (If I thought so, obviously, that would be a reason not to vote for John McCain.) It's because we want to discourage men from ditching their wives, and closing off certain very public avenues of advancement isn't a bad way to go about discouraging that sort of thing. The point is to deter misbehavior, not to protect the country from the perils of being governed by a rake.
Update: Okay, yes, "scot-free" is a little strong ...