Now THAT is a challenging question.
First let me agree with you. Our reporting suggested that the way Karl Rove and W won Ohio in 2004, providing their margin of victory over John Kerry, was by their exploitatioin of the gay marriage "issue" in the rural parts of the state, which turned out an unprecedented Republican vote that year.
I cannot think of another big issue of this kind on which opinion shifted so dramatically, so quickly. Slavery, a vote for women, prohibition, racial segregation--all of them only changed after years, decades. The repeal of prohibition became popular pretty quickly during the late 1920s, I think (missed it myself), but that wasn't such an emotiional issue by then.
The hard part of your question is what this might mean. I have been thinking, and arguing, for some time that we are living through a time of quite radical reinvention of American society. Dramatically improved tolerance for homosexuals is one manifestation of this; tolerance for inter-racial dating and marriage is another. But the biggest sign may be simple demographics: California is already a majority-minority state; more than half its residents are not white. This is a harbinger of the America to come. We are, once again, reinventing our country with new people and new values. We've done it again and again since 1776.
To my eye, the Tea Party and other manifestations of conservative opposition are signs of this as well. In a way I'd argue is pretty healthy, a lot of white Americans, especially, are showing us how anxious they are about the ways in which their country is changing. They don't want this new America I'm talking about here. They liked older versions.
But they're gone forever, as today's big event reminded us again. We have a half-black, half-white president of the United States named Barack Hussein Obama. How do you think your great grandparents would have reaced to that?
Oh, and by the way, he has just begun his second term.