But relationships so routine they often go barely noticed in everyday life still provoke fervent argument when they come under the glare of court challenges and the endless partisan fights of the culture wars. Particularly contentious: When the question of gay rights intersects with children or with government benefits and protections.
Should gays and lesbians be allowed to adopt children? In 18 states they can, and in some others the decision is left up to local judges.
Should they be protected from job discrimination? Twenty-one states say yes.
And what about the Boy Scouts' ban on gay leaders? Transgendered Girl Scouts? The idea that gay marriage will surely lead to further loosening of what society will accept?
Gay rights are regularly sliced into multiple wedge issues at election time. How gay marriage is folded into the presidential campaign will be a good barometer for how far America's views have shifted — perhaps far more than Obama's statement itself, though its appearance as the general election season begins is hard not to notice.
And in a society still divided, it cuts both ways. Ralph Reed, chairman of the conservative Faith & Freedom Coalition, said Obama had just handed presumptive Republican nominee Mitt Romney a choice culture issue.
"Twenty-four hours ago, we were talking about what Romney had to do to get social conservatives on board," Reed said Wednesday in a telephone interview. "Now, they're scrambling for a seat in first class."
Romney opposes gay marriage but says states should be able to decide whether to offer limited legal rights to same-sex couples. "This is a very tender and sensitive topic," Romney said Wednesday.
It's also a topic that can fire up subsets of voters without distracting most from the U.S. economic troubles. Gay rights issues are cited as the nation's most important problem by 1 percent or less of the population, according to Gallup.
As he presented his evolving view, Obama treaded cautiously. He acknowledged that many Americans have religious objections and said that influenced his decision not to endorse gay marriage earlier. He spoke in personal, not political, terms, noting his experience with the gay parents who serve on his staff or who are raising friends of his daughters, Sasha and Malia.
The girls would never imagine "that somehow their friends' parents would be treated differently," Obama said. "It doesn't make sense to them and, frankly, that's the kind of thing that prompts a change in perspective."
Presidents determined to push the nation forward on great social questions give rousing addresses from their bully pulpits. Franklin Roosevelt laid out four freedoms — freedom of speech and worship, and freedom from want and fear — as American ideals worth fighting for in Europe and ensuring for suffering people at home. Lyndon Johnson declared that "we shall overcome" must become the conviction of all conscientious Americans, not only civil rights marchers.
But for Barack Obama on Wednesday, it was different. Instead of proposing, he concurred. Instead of advocating, he acknowledged. Instead of saying, "We must," he was saying, in effect, "We already are."
New York Mayor Michael R. Bloomberg was quick to mark it a major turning point in history. "No American president has ever supported a major expansion of civil rights that has not ultimately been adopted by the American people," he said.
Others weren't so certain.
"He's not waving the flag. He's nodding his head," said political historian Evan Cornog, dean of Hofstra University's School of Communication, who has studied how presidents build their own story lines.
When historians look back in 50 years or so on the struggle for gay rights and its place in American culture, Cornog said, "It may be that 'Modern Family' means more than Obama's statement."