Let's take a look at how unthinkable the concept had been before 2004. Look at this cartoon from 1980:
Treats the whole concept as a joke, right? By 2015's standards that's rude. Now consider what kind of joke it was. Gahan Wilson specializes in cartoons of hideous monsters and eviscerated children. So it's a horror joke. And it appeared in Playboy, so a joke restricted to the sturdy-minded.
Andrew Sullivan wrote a serious essay proposing gay marriage in the New Republic in 1989. Even to that very liberal audience it was considered a radical idea. Sullivan said the most common response was laughter. It seemed to be one of the many wonkish proposals that never go anywhere.
Another example from 1997: in the Babylon 5 episode "Racing Mars" (s4e10) two men are given the identity of newlyweds. So it was an idea that audiences could handle . . . as being in place over two hundred years in the future. The audience for that was restricted to people who weren't bothered by the thought of people from two different species marrying, so again sturdy-minded. It was up there with the reference to a female Pope from the same season--an obscure joke to show how different things were in the future.
Then Gavin Newsom pushed it into the now. And eleven years later his counterpart Kim Davis is being sent to jail for the flip side of the same cause. This is staggeringly fast for a change in law and culture.
Look at how long it took to change the laws and attitudes on interracial marriage:
And that graph needs an update--the solid blue line just shot up to 100%.
Why did this happen so fast?
I think the biggest factor is that nobody's mind was changed. Public opinion on gay marriage can be divided into three groups--people who actively wanted it (But before 2004 considered it unattainable), people who don't object to it or consider it a "nice to have," and those who actively oppose it. There hasn't been significant movement among the groups.
With interracial marriage the size of the groups changed, mostly by the old generation dying off and new voters with different views coming of age. Eventually there was enough support that states began repealing their laws and judges were willing to take a stand for civil rights. This was driven by many other votes on civil rights acts and elections where politicians had to take stands on civil rights. Eventually it became clear the majority accepted it, even if they didn't like it.
Gay marriage was not majority vote driven. State referendums opposed it even in liberal states such as California. Politicians opposed it during elections only to reveal their support later (Obama did support it during his re-election campaign). More states had gay marriage legalized by judicial rulings than legislative votes. And the final legalization came from the Supreme Court.
What we're seeing here is that a lot of politicians and judges are in the middle group, considering gay marriage "nice to have" and willing to follow the lead of which ever group of activists currently has more clout. That doesn't settle the issue, especially when the final vote on it is 5-4. That tells the activists on the losing side "find one more vote," not "give up, you're outnumbered."
Kim Davis and her supporters are hoping her martyr act will start a preference cascade the other way. I don't see it happening. Instead we're going to have incredible sturm and drang over every upcoming Supreme Court appointment. Which will be a change from those being driven by abortion politics, another issue where majorities don't rule (if politicians had to live with the abortion laws they passed instead of expecting them to thrown out by judges we'd see more European-style compromises instead of grandstanding).
I'm all for gay marriage. It's a step forward for personal freedom. It lays the groundwork for more changes I'd like to see. I just wish I had been passed by referendums and legislatures instead of judges. Democracy is already being weakened by the incredible amount of power held by unelected bureaucrats enforcing laws. With unelected judges making new ones there's even less connection between what people want and what the government does. If that goes too far we'll lose democracy as a way for settling our differences . . . and I don't like the alternatives.