You could say that FiveThirtyEight's Nate Silver has a bit of a grasp on numbers. The man who correctly predicted the outcome of the 2012 election is somewhat of a statistical wizard (sometimes more literally than others).
Today, as people show support (for one side or the other) while the Supreme Court of the United States considers the constitutionality of California's Prop. 8, Silver published an in-depth statistical analysis of polling data that indicates that gay marriage doesn't even need SCOTUS support to be ready for 2016.
Silver's post walks you through the polls and data sets he chose to work with, explains that support for gay marriage isn't increasing at a faster rate than it was back in '04, and suggests that a national ballot referendum supporting gay marriage would probably go over successfully in 2016.
Thus, even if one prudently assumes that support for same-sex marriage is increasing at a linear rather than accelerated pace, and that same-sex marriage will not perform quite as well at the ballot booth as in national polls of all adults, the steady increase in support is soon likely to outweigh all other factors. In fact, even if the Supreme Court decision or some other contingency freezes opinion among current voters, support for same-sex marriage would continue to increase based on generational turnover, probably enough that it would narrowly win a national ballot referendum by 2016. It might require a religious revival among the youngest generation of Americans to reverse the trend.