In fact if you ask the question as to whether marriage between two people of the same sex should be a private decision, or whether it should be subject to government intervention about 63% of Americans think it should be a private decision. For comparison, about 95% think marriage between two people of different races should be a private decision not regulated by the government, while only 18% think the same applies to marriage between minors.
If you ask the question in the more typical framing of "should the government permit gay marriage?" the numbers are more around 42% in favor. The difference is that the traditional framing sees marriage as something the government bestows. The less-traditional framing sees it in a more libertarian light, that the government has no business in the private matters between two consenting adults.
Nate argues that framing the argument in future battles will help.
[A]dvocates for same-sex marriage can do a better job of framing their argument. Generally speaking, appeals to government noninterference are fairly popular; people don't like government telling them what they do and they don't have the right to do. Posit equal treatment under the law as the default -- how dare the government make a law that abridges this right on the basis of something as trivial as sexual orientation.
Meanwhile, in a review of a study of gay marriage views state-by-state, Andrew Gelman of Columbia argues for the idea of a tipping point. It seems that
In the past fifteen years, gay marriage has increased in popularity in all fifty states. No news there, but what was a surprise to me is where the largest changes have occurred. The popularity of gay marriage has increased fastest in the states where gay rights were already relatively popular in the 1990s..
That is, rather than seeing a general shift towards more favorable views, the rate of approval accelerates faster when there is already a certain threshold.
In the graph, you can see a huge increase of approval rates in states at the top of the list, which were already pretty positive in 1994. On the other hand, at the bottom of the list, there is much less change and in the religious theocracy of Utah, support has actually declined.
Is this because there are more gay people in the friendlier states, or more "out" gay people? Thus people in friendlier states are more likely to know gay people and understand how deeply injuring the marriage bans are. It's harder to discriminate when you can picture the victim of that treatment. Or is it because, once you reach a certain threshold, anti-gay views become more socially unacceptable, the way overt racism or other prejudices are unacceptable? Whatever the case, it's clear that a 50-state recognition of marriage equality is a long way away, and, as in many other aspects of social progress, including racism and women's rights, there is a rump of states that will lag behind. Sadly, they are mostly the same states in all these issues: the deep South evangelical-land.
Meanwhile, despite the changes in momentum, we've seen no progress federally, and if anything a regression this last week.
(Click the graph for a closer view).