President Barack Obama has called the same-sex marriage campaign in the United States as the "fastest social movement" in the nation's history.

Speaking before young leaders in London on Saturday, Obama said "in the United States what's been remarkable is the rapidity with which the marriage equality movement changed the political landscape and hearts and minds, and resulted in actual changes in law."

He said on marriage equality, he initially favoured civil unions as "labelling those partnerships as marriage wasn't necessary as long as people were getting the same rights, and it would disentangle them from some of the religious connotations that marriage had in the minds of a lot of Americans."

Obama said his children Malia and Sasha had the greatest impact on him that changed his view on marriage equality.

He said he was made to understand that it was not simply about legal rights but about the "sense of stigma."

"That if you're calling it something different it means that somehow it means less in the eyes of society," he said.

The U.S. president said in campaigning for same-sex marriage, the LGBT community presented it as not some radical movement but about families.

Americans for Truth president Peter LaBarbera said one factor why the pro-marriage movement lost the battle against same-sex marriage was their refusal to condemn homosexuality.

"The Christian and pro-family movement continually backed up and became afraid to state basic truths about homosexuality and right versus wrong," he told LifeSite News. "That only emboldened the 'gay' activist movement—and now our retreats embolden the transgender movement."

Activists focused too much on traditional marriage and not on the physical and mental consequences of homosexuality.

"Imagine if we declared: 'We will not talk about adultery.' People might logically conclude that there's nothing wrong with adultery. The same applies to the pro-LGBTQ Left's crusade to normalise homosexuality and gender rebellion," he said.