Some people cite David Vitter as an example of there apparently being different rules for Republicans and Democrats when it comes to sex scandals. I suggest there may be different rules when it comes to Senators and Representatives.
Let's look at an incomplete list of sex scandals.
Some Representatives:
*Chris Lee (R-NY) resigned the same day that Gawker reported that he was soliciting women in Craigslist personals, possibly because of impending news that he was seeking out transsexuals.
*Mark Souder (R-IN) announced his resignation on the same day he admitted to having an affair with a staffer.
*Eric Massa (D-NY) announced his resignation within a month after it was confirmed that the House Ethics Committee was investigating charges of sexual misconduct.
* Vito Fossella (R-NY) announced he would not run for re-election nineteen days after disclosing an affair that resulted in an illegitimate child.
* Tim Mahoney (D-FL) lost an election within weeks of reports that he had hired a former mistress to work on his staff as hush money.
* Mark Foley (R-FL) resigned within a day of news breaking about his Congressional page scandal.
* Bob Livingston (R-LA), all but assured of being the next Speaker of the House, resigned about a day after reports that Larry Flynt had dug up details on an affair.
* Mel Reynolds (D-IL) resigned less than two months after he was indicted (and eventually convicted) of sex with a minor.
Some Senators:
* John Ensign (R-NV) took almost two years after admitting to an affair and sixteen months after it was reported that the FBI was investigating him before he resigned from office.
* Larry Craig (R-ID) initially announced plans to resign days after the story of his "wide stance" broke, but took back his decision and stayed in the Senate for another sixteen months, although he declined to run for re-election. However, the courts declined to let Craig withdraw his guilty plea.
* David Vitter (R-LA) was found to have used prostitutes, but he didn't resign and Louisiana voters re-elected him three years later.
*Bob Packwood (R-OR) took almost three years after reports of sexual assault and abuse against multiple women before he stepped down, and perhaps only because the Senate was about to expel him anyways.
I hypothesize that Senators are slower than Representatives to step down in the face of scandal because their day of reckoning with the voters is usually further in the future and because they have more invested in their seat, as Senate races are usually much more expensive in House races and Senators have invested more time in their political careers than Representatives.
This doesn't mean that there isn't a double standard for Republicans and Democrats, just that any difference is exacerbated if you compare a Democratic House member such as Anthony Weiner to a Republican Senator like David Vitter instead of to a Republican House member.