By now, it's clear that Mark Sanford has about as much of a future in politics as he does in sumo wrestling. His confession of adultery was all it took to demolish any hopes he had of running for president -- and perhaps even to force him to step down as governor of South Carolina. But why? After all, we've had presidents who are revered by posterity despite being unreliable husbands. Hardly anyone even remembers now that Franklin Roosevelt had a mistress, that Dwight Eisenhower may have had one, or that John Kennedy had several. In the intervening decades, we've...