In one very, very specific sense there's not a moral equivalency between what we know about Rep. Anthony Weiner's behavior and that of Sen. David Vitter. And in that narrow specific, narrow sense it is not the case that if Weiner should resign then Vitter should resign as well.*
As it happens this narrow, specific sense is probably anathema to the conservatives who are clamoring for Weiner's resignation, but we already know they're fucking hypocrites and partisan assholes. The consequent fact that their moral opinions are worth exactly zero doesn't change the equation, however.
So here's the deal.
When a particular woman semi-randomly caught Rep. Weiner's eye he evidently sent them unsolicited photos of his bulgy underwear. Without prior agreement that's (social if not legal) harassment and sexual imposition without consent. And from a moral standpoint that's pretty objectionable whether or not the objects of his solicitations wound up appreciating his, um, attention.
Senator Vitters, on the other hand, did not courier unsolicited soiled baby-play-fetish diapers to semi-random women. Instead he hired and paid consenting adult sex workers agreed-upon sums to let him pretend to suckle milk from their breasts and to hold his feet high over his head while they unpinned his diapers, cleansed his soiled groin, and presumably "finished him off" with previously-agreed-upon manual, oral, or penetrative sex. And from a moral standpoint that's entirely unobjectionable in the sense that to the extent one could ask Rep. Weiner to resign one could not automatically demand Sen. Vitter to resign as well.
Frankly I believe Senator Shumer, Senator Reid, Minority Leader Pelosi* should stand up before their respective august bodies and, in the spirit of bipartisanship and fairness, recite my argument exactly.
Furthermore, in my own reach across the aisle I invite partisan Republican bloggers, pundits, and politicians to freely repost or reuse my points in their castigations of Weiner and their equally full-throated defenses of Vitter.
Because, no, really, seriously, in all honesty it really is narrowly and specifically far more immoral to mail unsolicited photos of one's underwear than it is to pay an informed, consenting adult to baby-wipe your ass and then jack you off while saying "ootchi, gootchi, goo naughty baby Davey."
* There are numerous other related reasons why Sen. Vitter should have resigned. And been castigated by his peers. And been voted out of office if he refused to resign. This just isn't one of them.
** Or possibly Sen. Frankin since I'm pretty sure he could do it with a straight face.