Rep. Barney Frank had an affair with a prostitute. He did not resign. I don't think he needed to because he did not try to justify what the other guy did, and because he had not made a big deal about sexual morality before that. His district has re-elected him many times since.
Sen. David Vitter had sex with a prostitute. He should have resigned. He had made a big deal about sexual morality before that - and still does, with no show of shame. He thinks the only thing he did wrong was getting caught.
The latest family values warrior caught with his pants down is Sen. John Ensign. He had an affair with a campaign staffer whose husband worked for him. He seems to have gotten their son a job, too. He only admitted the affair after he was blackmailed. Ensign was a particularly egregious hypocrite. In addition to being a family values culture warrior in general, he had specifically called on his Senate colleague Larry Craig to resign during his sex scandal. And Ensign called on Pres. Clinton to resign during his sex scandal.
It is getting to the point that if public officials makes a big deal about marriage, family, and sexual honor, it is easy to assume that they are sleazy.