On Wednesday morning it did just that, with the Supreme Court striking down not just the federal Defence of Marriage Act (DOMA), which defined marriage as between the two sexes, but also a Californian state law prohibiting gay marriage known as Proposition 8.

Now, says Professor Klarman, there is little doubt that within the foreseeable future gays will be able to marry in every state in the union. ''It is going to be all over in the snap of a finger,'' he says, before clarifying that he means in comparison to other social changes. A generation alive today will wonder that gay marriage was ever banned, he predicts.

Professor Klarman, author of From the Closet to the Altar: Courts, Backlash, and the Struggle for Same-Sex Marriage, tells the story of the Supreme Court judge Lewis Powell, who in 1986 was the deciding vote in support of a now infamous decision to uphold a Georgian anti-sodomy law.

While considering the judgment, Justice Powell told one of his clerks that he had never met a homosexual. He did not realise the clerk was gay.

If he had known, says Professor Klarman, he might have found differently. ''People want what is good for the people that they love or that they admire,'' he says.

As soon as gays started coming out, resistance to conferring upon them the rights enjoyed by fellow citizens began to wane. Further, once cases started to appear before the courts the momentum increased because while opposition to gay rights tends to be rooted in Old Testament doctrine, religious arguments cannot be mounted in court, says Professor Klarman.

On the forecourt of the Supreme Court, 1000 or more people stood in the heat and waited. When the news broke a cheer swept through the crowd. DOMA had just been struck down. A few minutes later Proposition 8 fell on procedural grounds. In scrapping DOMA, the court made it clear that states that do not recognise gay marriage will not be forced to. Nor did the court find a constitutional right of gays to marry.

But in the 12 states that do recognise gay marriage federal agencies will now recognise the institution, clearing the way for gays and lesbians to gain access to more than 1000 legal protections and benefits formerly denied them.

Gay marriage advocates say that the language in the decision striking down DOMA will assist them in fighting for the institution in states that do not yet recognise it.

In a 5-4 ruling, Justice Anthony Kennedy wrote: ''The federal statute is invalid, for no legitimate purpose overcomes the purpose and effect to disparage and injure those whom the state, by its marriage laws, sought to protect in personhood and dignity.

''By seeking to displace this protection and treating those persons as living in marriages less respected than others, the federal statute is in violation of the Fifth Amendment.''

Essentially the majority found the federal government cannot enact prejudicial laws without good cause.

In an angry dissent which he read from the bench - normally a sign of particularly strongly held views - Justice Antonin Scalia wrote: ''That is jaw-dropping. It is an assertion of judicial supremacy over the people's representatives in Congress and the executive.''

Outside the court Michelle McLeod, who was drummed out of the US Navy in the late 1980s for her homosexuality, stood and wept.

''I never thought we'd get gay marriage. I never even thought we'd get it in one state, let alone 12 and DC, and now the whole nation. I did not think it would happen in my lifetime,'' she said.

''The younger generation we have to thank for that [public change in attitudes towards gays]. They're more open-minded.''

Nearby, Michael and Shalom Konstantino, who are already married, were celebrating. They said the decision would have a huge impact on their lives, particularly in helping Shalom secure permanent residency.

''Not having our marriage recognised by the federal government - Shalom is a foreign national - this ruling means the world to us,'' said Michael, who met Shalom while he was working in the US embassy in Tel Aviv.

By early afternoon, Californian governor Edmund Brown said he had already acted to reinstitute gay marriage.

''Jesus wept'', tweeted the conservative former presidential hopeful Mike Huckabee.

The Republican House Speaker, John Boehner, said he supported America's system of checks and balances, but defended his party's decision to spend $US3 million defending DOMA in court.

''A robust national debate over marriage will continue in the public square, and it is my hope that states will define marriage as the union between one man and one woman,'' he said.

Minutes after the rulings were announced, Kris Perry and Sandy Stier, the plaintiffs who challenged California's anti-gay marriage law, were in the middle of a television interview when their phone rang.

It was the President. ''We're proud of you guys and we're so proud to have this in California,'' said Barack Obama. ''And it's because of your leadership things are heading the right way. So you should be very proud today.''

They invited him to their forthcoming wedding.