A new bid to win Assembly backing for legislation allowing same-sex marriage is set to collapse next week.
Stormont is to debate the issue for the second time in just over six months, but the motion will fail because the DUP has tabled a 'petition of concern'.
The mechanism means approval would require a majority of both unionists and nationalists – and with its 38 members the DUP can stymie the move.
The motion from Sinn Fein came after a so-called constitutional convention, which met earlier this month, recommended that same-sex marriage should be legalised.
The party's Caitriona Ruane, who attended the convention in Dublin, said that the 79% vote in favour was "a real cause for celebration". "This is an equality issue. People who are gay and lesbian, bisexual and transgender in our communities, they deserve rights.
"What the DUP need to do is go out and explain to the gay and lesbian people in their communities why they are blocking equality. Equality threatens nobody," the former Education Minister argued.
The DUP's Peter Weir said, however, the use of the petition device would ensure the motion would not be carried. "We are the only party to have a united position in opposition to the redefinition of marriage. However, we also do not believe that a constitutional convention in the Republic of Ireland should have any input into policy which is wholly a matter for representatives in this part of the United Kingdom to decide," he said.
"The debate on Monday will only result in further embarrassment for those parties and individuals who avoid telling the electorate where they stand."
The move came after both the Catholic and Presbyterian churches wrote to all 108 MLAs urging them to reject the Sinn Fein motion.
Last October a joint Green Party-Sinn Fein initiative insisting that same-sex couples "should have the same legal entitlement to the protections, responsibilities, rights, obligations and benefits afforded by the legal institution of marriage" was defeated by just five votes – 50 against to 45 in favour of the motion.
The DUP voted en masse against the call, while other parties were split.
Green Party leader Steven Agnew said the debate last year was "an important first step for the Assembly, not only on gay marriage but in debating an issue of specific interest to the lesbian, gay and bisexual community (which) in itself was significant".
The Assembly divided narrowly last October on a joint Sinn Fein-Green Party motion for marriage equality which was defeated by just five votes. The motion exposed internal divisions within the SDLP, Ulster Unionists and Alliance, with Sinn Fein and the DUP at loggerheads – the former voting in favour en masse and the latter entirely opposed.