And then there’s this argument, which is good, but not mine:

Dear Jonah,
Your correspondent misses the point of morality-related resignations entirely. Most Conservatives aren’t urging Vitter to resign because they hold him to a higher moral standard – it’s to spare them the painful, extended humiliation of a six-month public airing. Does he really imagine that Democrats wouldn’t have been thrilled by a Jefferson resignation? Does he think that Jefferson (or the Democrats) got away with something because he refused to resign?
There is some truth to the higher moral standard bit, I suppose – if you’re going to talk about morality until everyone’s sick of hearing about it, you should probably make an attempt to behave morally – but that’s not the point of the resignation. It’s all about putting the party first.