U.S. state votes legalizing same-sex marriage show that Americans are rapidly moving to the "right side of history," U.S. Ambassador David Jacobson says, likening it to the battle for civil rights for African-Americans.

"I have never seen an issue in the United States where the public has flipped as quickly as they have with respect to same-sex marriage," Mr. Jacobson said in an interview Wednesday, one day after voters in Maine, Maryland and Washington voted in favour of legalizing same-sex marriage in their states.

Only a few years ago, same-sex marriage was an issue that U.S. conservatives were putting on state ballots to get their base to the polls on voting day, he said. Now, for the first time, advocates of same-sex marriage are winning.

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"In 2004, which is eight years ago, George W. Bush put same-sex marriage on the ballot…in what they viewed as battleground states, so that it would get Christian conservatives out to the polls. And it worked," Mr. Jacobson. "In every place that it was on the ballot, it won – outlawing same-sex marriage."

Some courts issued decisions allowing same-sex marriage, but that sparked a cry that judicial decisions were going against the will of the public. Then some state legislatures moved to legalize same-sex marriage. Six U.S. states recognize same-sex marriage, including New York, Massachusetts and Iowa. But Tuesday night marked the first time that voters passed ballot initiatives to legalize same-sex marriage in their states.

"Not only is there an incredible change in the public perception, but it proved something to me that I have believed for a long time," he said. "Which is that issues which support more and more freedom are on the right side of history."

Eventually those views win out, he said.

"Sometimes they take a long time, like racial issues in the United States, which took hundreds of years, and a civil war, and God knows how many court decisions, and firehouses in Selma, Alabama, to overcome.

"This is one of those ones where being on the right side of history can happen much more quickly. And I think it's very important."