In 2008, there were 8 Senators that supported gay marriage.
In 2010, there were 16 Senators that supported gay marriage.
Today - with Senator Bill Nelson of Florida announcing his support - there are now 51 Senators that support gay marriage. A simple majority.
Nate Silver built a complicated model based off of each individual Senator's voting record and how it moves with shifting public preferences. His result is the picture above. The trend line suggests that the days of anti-gay marriage sentiment are numbered; but his model also suggests that the number of senators recently announcing support for gay marriage falsely implies that this cultural war victory will happen sooner rather than later. Note that the Actual has surpassed the Projection.
Nate Silver is a better statistician than I, but I have an easier way to explain the current announcement of Senators that support gay marriage: Support for gay marriage has hit the Tipping Point.
The innovators were those who announced their support prior to 2008. The Early Adopters were those who announced their support between 2008 and 2010. We just hit the Early Majority and the Late Majority is on its way.
Even if the United States as a whole is moving very quickly towards being pro-gay marriage, the United States Senate is composed of two members of 50 different republics. Urban voters, who are likely to support gay marriage, are concentrated into a handful of these republics in the Northeast and the West Coast. While the more rural republics, in the South and Great Plains, are much more conservative in nature. Those states, and their representatives, will be laggards in adoption of this innovation. With a super-majority needed to break a filibuster in the Senate, it still might be a while before the US Congress takes on the issue.
I also sincerely doubt that any Republican Speaker of the House would be willing to put the measure up to a vote. No good can come from putting its members on record on where they stand on this issue.
Time for an analogy: By the end of 1944, it had become all but certain that the US was going to defeat the Japanese in the Pacific. The only questions left were when and how. That's where gay marriage advocates stand today. When and how.
Update: Sen. Heidi Heitkamp (D-ND) and Sen. Joe Donnelly (D-IN) just announced support for gay marriage. That brings the total up to 53. I still doubt that we will reach the threshold of 60 by the end of this Congress and that Boehner will bring it up for a vot.