Representative Anthony Weiner had to resign for a simple, pragmatic reason: Nearly every major leader in his party, from President Barack Obama and Representative Nancy Pelosi on down, agreed publicly that he should. He had become a liability. Every day that his scandal dragged on, Congressional Democrats lost more face for their inability to rein him in and shut down the circus surrounding him.

It’s true that Mr. Weiner’s peccadilloes barely distinguish him among today’s politicians. From David Vitter’s prostitutes to John Edwards' and Arnold Schwarzenegger’s love children to Gavin Newsom’s and John Ensign’s extramarital affairs with their staffers’ wives, there is no shortage of political leaders behaving badly these days. The list of errant civil servants has grown so long that it sometimes seems easier to tally those who have not strayed than those who have. So why do some survive, while others get run out of town on a rail?

The answer largely boils down to political realities. The survivors tend to be crucial to their party’s success, and thus, their pleas for forgiveness are answered with in-house backing. This was true with Senator Vitter, whose seat would have been filled by a Democratic appointee had his fellow Republicans forced him out. The straying politicians who cannot survive generally are dispensable or have mangled their mea culpas so badly that they make it politically impossible for colleagues to stand by them.

Representative Weiner did this, with his lies, obfuscations and awkward apologies. The fact that he claims never to have consummated his Internet affairs seems almost incidental in view of the constant stream of new revelations of his sexually compulsive, self-destructive online behavior. His claim probably also comes as cold comfort to his wife, a pregnant newlywed no doubt humiliated by the photos of her husband’s unmentionables circulating through cyberspace.

All of which leads to the more crucial point about these powerful men behaving badly: The exact details of their infidelities matter less than their shared pattern of lewd, reckless behavior coupled with a belief that the rules do not apply to them.

Scandal-plagued politicians often argue that the failure to honor vows to spouses says nothing about the ability to honor oaths to voters. But human nature has never been that neat or tidy. Who we are as spouses, parents, children and friends inevitably informs how we act in the public square. Voters elect human beings as their representatives, not walking platforms. And the way those human beings conduct themselves in private, when no one is watching, says a great deal about the value of their word in public. Character, unfashionable a concept as it may be, still counts.

That so many politicians today seem prone to flunking the fundamental character test of marital fidelity and sexual integrity is no reason to lower our standards for them, any more than a politician’s pragmatic value to his party excuses immoral or illegal behavior. It is simply a reminder that politics attracts troubled, egotistical individuals as well as mature, honorable ones. And voters have every right to give candidates a thorough vetting before bestowing the privileges that come with power.