If the reason isn't related to homophobia, I don't know what the reason could be. Both Republican Senators have admitted to illegal behavior related to sex which clearly also violates family values as I understand it since both men are married.
Sen. Larry Craig pleaded guilty, a plea he now wants to withdraw, to lewdly wiggling his feet and suggestively moving his hand inside a Minnesota airport restroom in what has been reported to be clear signals of being willing to have sexual contact. Apparently these bathroom hookups send terror through the hearts of some straight men.
Sen. David Vitter admitted publicly to "sinful" behavior which also happens to be illegal after his phone number was connected to a prostitution ring.
Would the response be different if Sen. Craig had pleaded guilty to disorderly contact after putting the moves on a woman in the airport? Would that have been an acceptable crime? Would the response have been different if Sen. Vitter's number had been found in the possession of someone who ran a male prostitution ring?
If one senator must go but not the other, that definitely has meaning. Maybe I've missed something and prostitution has become a family value and a Republican value.