Ross Douthat does a pretty good job pinning down why Americans afford their politicians so little breathing room for their personal life:

But by turning their personal choices to political ends, politicians lose the right to complain when those same personal lives are subject to partisan critiques. They can and should contest these critiques, but they can’t complain about them. In a culture as divided about fundamental issues as our own, the kind of weird attacks that Rick Santorum is enduring come with the vocation he has chosen.