Now that South Dakota's retiring senator Tim Johnson has announced his support for gay marriage, most news sites are reporting that there remain only three Senate Democrats who have yet to jump on the marriage-equality bandwagon. But there are really only two.
The confusion is a result of the hazy position of Louisiana's Mary Landrieu. Last Thursday, she said in a statement that, while her "personal views have evolved," the "people of Louisiana have made clear that marriage in our state is restricted to one man and one woman" and she intends to "support the outcome of Louisiana's recent vote." The next day, Landrieu ditched the evolution rhetoric and told CNN "that she personally believes 'people should love who they love and marry who they want to marry,' but that her obligation rests with the people of Louisiana who elected her."
Landrieu is making a legitimate (if politically calibrated) distinction between her personal beliefs and the stance she's taking as a representative of her constituents. And since there are no official rules for judging what, exactly, "supporting gay marriage" entails, you could feasibly place her on either side of the marriage equality debate.
But Landrieu, along with every other senator, won't actually have the opportunity to vote on gay marriage for the foreseeable future. A constitutional amendment banning gay marriage won't be coming to the floor of the Senate ever again. With the GOP in control of the House, a repeal of DOMA isn't going anywhere either, and if the Supreme Court strikes DOMA down (as seems likely), Harry Reid sure isn't going to try and reinstate it.
With her Official Voting Position largely if not completely irrelevant, Landrieu's personal beliefs are really all there is. Furthermore, personal beliefs, and not voting intentions, is the metric used to gauge the positions of senators who say that marriage should be left to the states. (Follow-up question: "But where do you stand personally?") As far as we're concerned, Joe Manchin and Mark Pryor are now playing for the championship.