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 also become far more aware of just how common such behavior is among officeholders—not only Gary Hart, Bill Clinton, and Rudy Giuliani but lesser-known mayors, governors, congressmen, and water commissioners. Nowadays, finding that a politician breached his marital vows is about as surprising as learning that a professional athlete failed a drug test. If Alex Rodriguez and Jason Giambi can overcome their guilt, why not Sanford?

Other politicians have survived—most conspicuously our 42nd president. In fact, you'd think the Monica Lewinsky scandal would have settled the issue once and for all. Democrats found themselves excusing Clinton's conduct, and Republicans who condemned it wound up on the losing side in both public opinion and the impeachment battle.

When it was over, Clinton left office with a 65 percent approval rating. Trust him with your daughter? Not a chance. But your economy? Sure.

George W. Bush, by contrast, finished his term with an approval rating of 22 percent. Trust him with your daughter? Sure. But keep him away from the economy! Both parties could have drawn the same conclusion: Voters have more important things to worry about than their leaders' sex lives.

Yet here we are again, disqualifying a possible White House aspirant because he couldn't keep his pants on. After two decades of high-level political sex scandals, we seem to have reached a consensus that marital fidelity is actually pretty important in a leader. Given the choice, we would prefer peace and prosperity to presidential rectitude. But we really want all three, and we think we can have them.

Call it residual puritanism or an overdose of religion if you want, but most Americans think wedding vows are not to be disdained. In recent decades, sexual mores have gotten considerably more relaxed, with one major exception: extramarital affairs. A 2009 Gallup poll found that 92 percent of us think adultery is "morally wrong"—which presumably means there are a lot more people who commit it than defend it. Only 40 percent of Americans think premarital sex is morally wrong, and only 47 percent say that of homosexual relations.

So Barney Frank's career survived his romp with a male prostitute, while John Edwards' fling with a campaign aide made him politically radioactive. Sex without marriage is OK. Sex in violation of marriage is not.

Why not? Because adultery, unlike a frisky bachelor lifestyle, connotes a reckless dishonesty at odds with our basic notions of integrity. Because it shows a lack of respect for the most important commitment that most of us will ever make. Because it indicates that the adulterer will always place his selfish desires above those who depend on him.

There is a cost to this approach, obviously. It disqualifies some smart, dedicated, and able people merely because they suffer a single flaw—and one that apparently is pretty common among the politically ambitious.

But so what? A talented executive can expect to lose her position for a single act of embezzlement. An outstanding journalist may be banished from his profession for one incident of plagiarism.

Of course, those lapses bear directly on how the offenders do their jobs, which is not the case with a governor who strays. But we don't vote for CEOs or newspaper reporters, which means they don't embody our higher aspirations. Americans think those elected to positions of public trust should have enough regard for the public to conduct themselves in an honest, upright way even in matters unrelated to their official duties.

Is it naive of us to believe that a politician who keeps his commitments to his wife will also keep his commitments to us? Probably. But not as naive as thinking that if he betrays her, he'll treat us any better.