LONDON — One of the most prominent opponents of gay marriage in Northern Ireland openly admits that he is “on the wrong side of history.”

Like King Canute, Mike Nesbitt, the leader of the center-right Ulster Unionist Party, knows he cannot turn back the tide of progress.

His colleagues in the hardline Democratic Unionist Party aren’t so sure.

They vetoed an historic parliamentary vote to legalize gay marriage Monday.

Soon afterwards, a DUP heavyweight told The Daily Beast that Northern Ireland would not accept defeat in the culture wars without a fight—after all, the province has held out for almost 50 years since abortion was legalized in the rest of the United Kingdom.

“Northern Ireland is the only part of the U.K. that doesn’t have the 1967 Abortion Act. So, this is not a unique situation,” said Jeffrey Donaldson, the DUP’s chief whip in Westminster. “Our position on marriage is that marriage is between a man and a woman.”

The party overruled a majority vote in the Northern Ireland Assembly that would have made same-sex marriage legal, as it is in the rest of the United Kingdom and the Republic of Ireland.

They were able to override the vote by using a measure that was introduced after the 1998 Good Friday peace agreement in order to safeguard the human rights of minority groups after decades of violence.

Opponents have accused the DUP of abusing the powers granted by the Petition of Concern system, which demands a supermajority including 40 percent of nationalist representatives and 40 percent of unionist representatives.

Donaldson, the party’s longest serving Member of Parliament, said people who reject gay marriage were a legitimate minority group worthy of protection.

“The Petition of Concern is to protect the interest of all minorities in Northern Ireland, and we believe that there is a substantial element of our society that believes in traditional marriage, so we believe they have the right to have their views upheld as well,” he said.

The Catholic Church offered a rare show of support, joining the Protestant unionists in calling for gay marriage to be rejected.

Monday’s vote was the fifth occasion on which representatives of the Assembly have voted on gay marriage. By a single vote, 53 to 52, they approved gay marriage.

Despite the hardline unionist’s veto, LGBT campaigners welcomed the Assembly vote as a crucial step towards acceptance.

Naomi Long, deputy leader of the Alliance Party, wrote on Twitter: “The DUP may have won the day, with abuse of a POC, but they lost the vote & are losing the argument.”

That argument is now headed for the courts. Two challenges to Northern Ireland’s ban on same-sex marriages will reach the courts in Belfast by the end of the year.

“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 U.K. and Ireland,” said Patrick Corrigan, Amnesty International’s Northern Ireland Program Director. “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.”

Earlier this year, a Northern Irish judge ruled in favor of gay-rights campaigners who sued a Christian bakery for refusing to decorate a cake with an image of Bert and Ernie from Sesame Street and the slogan “support gay marriage.”

Whether the law-change comes through the Assembly at Stormont or a court of law, Jayne Robinson and Laura McKee are engaged to be married as soon as the they are allowed to do so.

They have invited every politician who voted for gay marriage to attend the celebration. “It’s really positive that today our politicians have said yes, we are equal—and that’s definitely a positive step in the right direction,” said Robinson.