After the House of Commons voted to allow gay marriage in England and Wales last night, supporters of the traditional definition of marriage are looking to the House of Lords to reject the Government's bill.
"Peers should have confidence to vote against the Marriage (Same Sex Couples) Bill at its first House of Lords debate due on 3 June," said the Society for the Protection of Unborn Children.
The House of Commons passed the bill by 366 votes to 161. It was opposed by 133 Tory MPs.
Paul Tully, SPUC's general-secretary, said the consequences of allowing the bill to pass are "grave".
"Marriage will be redefined as a genderless institution. The link between marriage and child-rearing will be seriously eroded," he said.
"Once parliamentarians decide to dismantle legal protections for the institution of marriage, then the welfare of children – the main reason for state recognition of marriage – will suffer. Statistics show that the family based on the marriage of one man and one woman is the most protective environment for children, both before and after birth.
"It is revealing that neither Maria Miller, the minister in charge of the bill, nor Yvette Cooper, the Labour party spokesman, made one single mention of children in relation to marriage in this evening's debate.
"The experience of other countries where marriage has been redefined shows that calling same-sex unions 'marriage' damages real marriage – leading fewer people to regard marriage as relevant to parenthood, and all that that entails for tomorrow's children."
The Coalition for Marriage, which garnered 600,000 signatures to its petition defending traditional marriage, accused David Cameron of being a "marriage wrecker".
The coalition's campaign director Colin Hart, said: "This Bill is no longer about redefining marriage, it's now about wrecking marriage.
"The addition of Labour's £4bn idea to open up civil partnerships to heterosexuals proves what we've always said, this bill will unravel and undermine marriage.
"A civil partnerships free-for-all will create two-tier marriage, with the option of marriage-lite for millions of unwed couples.
"If he thinks he got a rough ride in the Commons, just wait for the Lords. It will be a long, protracted battle which will only remind the voters that the Government isn't listening.
"Cameron once said he's a 'marriage man'. No, he's a marriage wrecker."
As MPs debated the bill in the House of Commons hundreds of Christians were praying outside.
The prayer gathering was organised by Christian Concern and joined by MPs and Bishop Michael Nazir-Ali.
Also present were graduates of Christian Concern's Wilberforce Academy, a programme which seeks to equip young Christians to speak of Jesus Christ in public life, were also present.
One of the graduates, Emmanuel Boateng, said marriage "is a beautiful thing and a sacred thing and it should not be changed".
Andrea Minichiello Williams, chief executive of Christian Concern, said the prayer gathering sent the message to MPs that Christians "care passionately about marriage because it is good for our country".
"Several MPs made reference in the House today to the 'chilling effect' that equalities legislation has had on Christians in public life," she said.
"The ordinary Christian men and women who gathered here today are the kind of people who will be affected by the destruction of a common understanding of marriage.
"Marriage is between one man and one woman and our country's laws should continue to reflect this. It's good for men and women, good for children and good for society."