If you make it an issue, it's an issue
Why? One word: hypocrisy. As ABC News notes, Gingrich "constantly espous[ed] family values even while he carried on an affair" and "linked his party to wholesome family values and Democrats to, well, something else." In a similar vein, PoliBlog's Steven Taylor offers a more detailed explanation of the story's significance: "[W]hile I understand that the impeachment process aimed at President Clinton was itself about perjury and so forth, the underlying context was that of an extramarital affair. For Gingrich to see no hypocrisy in this overall situation is to be engaging in self-delusion and serious rationalization."
If politicians aren't lecturing voters about family values, they retain a legitimate argument for keeping their private lives private. But if they choose to portray themselves or their parties as defenders of personal moral virtue, it's only fair for observers to point out their potential shortcomings on that front.