On the eve of a Supreme Court decision that could decide same-sex marriage in America, nearly three-fourths of Millennials support it -- even as only 16 percent of adults aged 18 to 29 are actually married themselves. The latter is the lowest it's been in at least 10 years.

And it's more than a little ironic that marriage has become the defining political issue for a generation that's overwhelmingly single and perhaps less interested in marriage than those who came before it.

Millennial support for same-sex marriage has always been higher than for older generations, but in recent years, the gap has widened noticeably. In 2005, 5 percent more Millennials supported gay marriage than Generation Xers, according to the Pew Research Center; today, the difference is 14 percent. In 2005, 13 percent more Millennials supported it than Boomers; today, the gap is 28 points. Millennials are indeed a huge part of the reason public opinion on marriage has flipped so quickly.

Their evolution on the issue is also something that's played out in an especially Millennial way: online. The increased visibility of gays and lesbians is thanks in part to the Internet, which allowed the closeted to find a community and made it easier to come out. And a generation of Facebook users will forever have a red equal sign in their profile pictures album from 2013. Scrolling through sites popular with Millennials like Tumblr, it can be easy to forget there are still people opposed to same-sex marriage.

Millennials have been chided for their "slacktivism" before, but on same-sex marriage, there isn't much most Americans could do, at least at the ballot box. Less than a dozen states where it's legal were actually the result of a referendum or legislative action rather than a court order, and candidates' positions on the issue haven't necessarily led to changes in policy. Instead, they often follow when it has become safe and widely accepted. Social media became an outlet to express support in lieu of being able to vote for something.

But maybe there is some FOMO (fear of missing out) there too. The same anxiety of missing out when scrolling through Instagram can be in play in our politics. It's not uncommon to see Millennials compare LGBT rights to civil rights struggles of the past, and posting about it can feel like a way to be a part of that tradition -- a way to be "on the right side of history."

That might be why an issue that directly affects a relatively small number of people has received outsized attention. About 0.3 percent of Americans are actually in a same-sex marriage, according to Gallup, but the fight over same-sex marriage was always about more than just marriage.

And it might just be why a generation in no rush to tie the knot themselves is so enthusiastic about making sure others can.