It's the hypocrisy, stupid
I don't actually care what people get up to in the privacy of their own marriages or elsewhere with one or more consenting adults, and am prepared to feel sympathetic if they overstep, get caught out, and suffer public humiliation.
But when someone tries to impose religious and ethical values on me by writing them into law, they should expect me to assume that they at least have those values themselves.
The moment for David Vitter to stop pushing his religious/marriage agenda was the moment, whenever it was, that he himself acted in a manner that violated the sanctity of marriage, an institution he claims to consider sacred.
One reason people were willing to forgive Bill Clinton for his Oval Office assignation is that he never tried to lecture others about sex and infidelity. That didn't make his infidelity okay, but it meant he wasn't a hypocrite. And it helped that he compartmentalized well: his private failings didn't seem to have much effect on his ability to execute his public duties.
Many social conservatives actually live their values. But a distressing number of them publicly profess one thing while living another. Another (small-bore) example emerged yesterday: The arrest of Florida State Rep. Bob Allen (and state co-chairman for the McCain campaign) for soliciting a sex act from a male undercover officer. Allen received an "A" from the Christian Coalition in 2005-06, based in part on votes in favor of interfering in the Schiavo case and making "In God we Trust" the state motto.
Anyway, give the link a read.
Vitter, Bob Allen, politics, midtopia