Of the 105 MLAs who voted today, 53 voted for and 51 voted against gay marriage. Ulster Unionist leader Mike Nesbitt abstained.
But today's motion was doomed to failure because the DUP tabled a 'petition of concern', meaning the vote required separate majorities of both unionists and nationalists to succeed.
Patrick Corrigan, Amnesty International’s Northern Ireland Programme Director, said the vote is a "significant milestone on the journey to marriage equality".
But he added: "The abuse of the petition of concern, to hold back rather than uphold the rights of a minority group, means that Stormont has once again failed to keep pace with equality legislation elsewhere in the UK and Ireland.
"The battle for equality in Northern Ireland will now move to the courts, where same-sex couples have been forced to go to secure their rights as equal citizens in this country."
Two court challenges to the ban on same-sex marriages will be heard in the courts in Belfast in November and December.
Last night there were indications that up to four SDLP MLAs who have abstained or been absent in past votes are prepared to vote for the proposal now. One Alliance MLA, Trevor Lunn, who has voted against and abstained in the past, confirmed he will be supporting the motion.
However, one Ulster Unionist who voted for the proposal last time, Danny Kinahan, is no longer in the Assembly and his replacement, Adrian Cochrane-Watson, is believed to be opposed.
Three other unionists - NI21 leader Basil McCrea and independents John McCallister and Claire Sugden - are expected to vote in favour again.
Mr Lunn was one of three Alliance party representatives who abstained in the last vote, despite the official party line being to support marriage equality.
Yesterday he said the issue of same-sex marriage had not even been included in the party's last Assembly election manifesto. He said he had been "on a journey" over the issue. But he told the BBC's Sunday Politics programme that some of those who had praised him for voting against the measure in the past were now "quite critical" of him.
The Lagan Valley MLA said he respected the views of colleagues who he described as "stragglers" but he now regarded same-sex marriage as an equality issue.
"I think politicians are entitled to change their minds," he said. The SDLP's Claire Hanna revealed a number of her Assembly colleagues have also changed their minds - but refused to say how many. "People are balancing their consciences with their duties as elected representatives. But it is ridiculous we are the only region in the UK where same-sex marriages are not allowed," she said.
Today's motion, brought by both Sinn Fein and SDLP MLAs, is that "this Assembly calls on the Executive to table legislation to allow for same-sex marriage".
Mr Cochrane-Watson told the Belfast Telegraph earlier this year he remained "slightly uncomfortable" with the idea of gay marriage.
"I come from a Presbyterian family, from a very Christian household, and within my church and my family I'm just uncomfortable with it," he said.
"But, as I said, I'm committed to being respectful. Bear in mind, it was our party which was the creator of Section 75 (which enshrined the rights of every citizen to be treated equally in Northern Ireland) in the Good Friday Agreement."
Peter Lynas from the Evangelical Alliance said examining potential reform of civil partnership legislation was a better way to address concerns raised by the LGBT community.
"The Evangelical Alliance supports marriage between a man and woman as it's understood in the majority of countries around the world," he said.
"If the latest motion in Stormont is really about equality then we are happy to have a discussion about how you fix the civil partnership legislation - we think that's the way to do it."