Human rights charity Amnesty International says the Northern Ireland Assembly will not be able to block same-sex marriage indefinitely.
The Assembly has been debating the issue on Tuesday – despite attempts by the Democratic Unionist Party to kill off any prospect of a vote.
Patrick Corrigan, Director of Amnesty International, said: “Politicians in Northern Ireland who continue to block marriage rights for same-sex couples are like latter-day King Canutes, trying in vain to hold back the tide of equality.
“States may not discriminate with regards to the right to marry and found a family, on the basis of sexual orientation or gender identity.
“That obligation is clear in international law.”
DUP Chief Whip Peter Weir defended his party’s opposition, saying “Same-sex marriage is not an issue of equality or human rights”
Sinn Fein, the Social Democratic and Labour Party (SDLP), the Alliance Party, NI21 and the Greens, all support equal marriage.
The DUP has continually resisted supporting LGBT equality across the board. First Minister Peter Robinson and Health Minister Edwin Poots have refused to lift Northern Ireland’s lifetime ban on gay and bisexual men giving blood.
Demonstrations by equal marriage campaigners took place in Northern Ireland last month on the eve of the first same-sex marriages in England and Wales in order to highlight the equality gap in the province.
Same-sex marriages conducted in England and Wales are currently only recognised as civil partnerships.