A controversial proposal to extend marriage rights to same-sex couples in Northern Ireland has been rejected by the Assembly.
The Sinn Fein motion was voted down during a heated debate at Stormont.
Opponents of change said the state should not meddle in the religious ceremony of marriage but advocates vowed that one day the region would be brought into line with many other countries.
DUP MLA Mervyn Storey said: "The re-definition of marriage would represent a change of monumental significance. It must not happen in Northern Ireland."
Northern Ireland is the only part of the UK without marriage rights and Amnesty International has warned there could be a legal challenge.
This is the third time the matter has been brought to a vote and defeated following a veto by the largest party, the Democratic Unionists. MLAs voted 51 to 43 against the proposal.
Patrick Corrigan, Northern Ireland 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."
The Democratic Unionists tabled a contentious voting mechanism ahead of the debate, meaning the motion could only pass if a majority of both unionists and nationalists support it.
With the DUP holding most unionist seats, the party's decision to table a petition of concern means the move to replicate laws already introduced elsewhere in the UK is doomed to fall.
Supporters of gay marriage held protests in Belfast and Londonderry to object to the use of a petition on such an issue.
The voting mechanism was incorporated into Assembly structures during the peace process to protect minority views.
Mr Corrigan said: "With politicians continuing to block equality, it is now inevitable that same-sex couples in Northern Ireland will take a legal challenge on the basis of inferior treatment with regards to the right to marry and found a family."
He added: "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. This means that marriage should be available to same-sex couples in Northern Ireland, just as it is now in England and Wales and will shortly be in Scotland."
An Ulster Unionist minister in the power-sharing administration, Danny Kennedy, said his opposition was consistent with the teachings of religious scripture.
"The separation of church and state becomes of extreme importance. The church should never be the slave of the state and consequently the state has no right to dictate the terms of religious marriage to the church.
"In my view it is neither sensible nor desirable to allow the state to interfere in the religious institution of marriage simply for political convenience."
Nationalists supported change despite the head of the Catholic Church, Cardinal Sean Brady, voicing his opposition.
SDLP MLA Dominic Bradley said his party was strongly in favour of the right to same sex marriage being enshrined in legislation.
"The motion recognises that the state does not have any role in dictating to religious groups which ceremony they can and cannot conduct."
The first gay marriages took place in England and Wales last month. Scotland passed a similar law in February and the first same-sex marriages are expected there in October.
In Northern Ireland, with a greater proportion of Catholic and Protestant churchgoers than other parts of the UK and arguably a more conservative social culture, any change to the law would prove highly controversial.
A referendum on the issue is likely to be held in the Republic of Ireland next year.
Sinn Fein MLA Megan Fearon said equality was an absolute necessity.
"Equality is an all or nothing concept. You cannot be a little bit equal to someone else, that is simply not how it works.
"Civil partnerships are not enough."
Catholic bishops had urged politicians to reject "marriage equality" for same-sex partnerships in Northern Ireland while the Church of Ireland restated its opposition to any change to the traditional definition of marriage.
The motion called on the DUP Minister for Finance and Personnel, Simon Hamilton, to introduce legislation to guarantee that couples of any sex or gender identity receive equal benefit.
The motion also stated that religious institutions should have the freedom to decide whether to conduct same-sex marriages.