Here's a look at data from Washington Post-ABC News polling on the question since 2004:
And, here's a chart detailing Pew polling on support/opposition to gay marriage since 1996:
It doesn't take a professional pollster to understand what the chart tells us. In 1996, as the country was easily re-electing Democratic President Bill Clinton, two-thirds of the public opposed gay marriage. Sixteen years later, as the country was re-electing Democratic President Barack Obama just 43 percent opposed it while 48 percent favored it.
Go deeper into the Pew numbers -- and thanks to Pew, you can! -- and you see why those trend lines won't be reversing themselves. In 2011-2012, 62 percent of people 18-29 supported gay marriage -- by far the strongest support among any age group. During that same time period, just 32 percent of those 65 or older supported gay marriage.
The simple truth: Support for gay marriage tracks directly with age. The younger you are, the more likely you are to support it. Given that, it's hard to imagine gay marriage getting less popular as the years go on.
Whatever the Supreme Court does with its two gay marriage cases next year, the die has been cast on the politics of the issue. By the 2016 presidential election, this could well be a decided issue that neither party -- yes, that includes Republicans -- spends much time talking about.