Religious campaigners will try to make gay marriage a major issue in marginal constituencies during the next election, according to a statement released today.
The threat, from the anti-abortion group Society for the Protection of Unborn Children (Spuc), comes after David Cameron confirmed he would ensure same-sex marriage legislation is passed before 2015, when the country goes to the polls.
"Spuc and its colleagues in many pro-family, Christian and Muslim groups will ensure same-sex marriage becomes a big general election issue, especially in marginal constituencies," chief executive John Smeaton said.
"Mr Cameron's speech reveals that his understanding of marriage and religion is woefully simplistic and ignorant.
"His mantra of 'equality' totally ignores the nature, history and role of marriage, which is the union of one man and one woman ordered towards the procreation of children."
Observers had begun to suspect David Cameron was going to back down on the legislation amid vocal criticism from the Church and his own party members, but he reaffirmed his commitment to the reform at a lesbian, gay, bisexual and trans-sexual community party in Downing Street last night.
"I just want to say I am absolutely determined that this coalition government will follow in that tradition by legislating for gay marriage in this parliament,” he told attendants.
"I think marriage is a great institution. It's something I feel passionately about and I think if it's good enough for straight people like me, its good enough for everybody and that's why we should have gay marriage and we will."
The prime minister also criticised sections of the Church for their continued opposition too the policy, warning that they risked closing the door on potential supporters in the same way the Conservative party had done in the 1980s.
"It locked people out who were naturally Conservative from supporting it and so I think I can make that point to the Church, gently," he said.
Parliamentary observers had wondered if the government was about to back down on gay marriage after it failed to appear in the Queen's Speech.
Downing Street then promised to give Tory MPs a free vote on the policy, as a matter of conscience.
Opponents said that was an unnecessary complication to what was essentially a change to civil law, but the policy almost certainly has support from Labour, Liberal Democrats and enough Conservatives to pass.
Nevertheless, the government's decision to push ahead with the legislation is irritating many backbench Tory MPs, who are feeling newly confident after their massive rebellion over House of Lords reform.