Christians have said they will continue to defend the traditional definition of marriage following last night's Commons vote on the Marriage (Same-Sex Couples) Bill.
The Bill was backed by a majority of 400 to 175, despite more than half of Conservative MPs refusing to give their support.
Prime Minister David Cameron said the legislation was "about equality".
"But it is also about making our society stronger," he said.
"I know there are strong views on both sides of the argument - I accept that. But this is an important step forward for our country."
Deputy Prime Minister Nick Clegg welcomed the outcome of the debate, saying Britain would look back on it as a "landmark for equality". Labour leader Ed Miliband said it was a "proud day" for Britain.
Christians have raised serious concerns about the implications of the change for society and religious freedom.
Director of advocacy at the Evangelical Alliance, Dr Dave Landrum, said the Government had "no right and no mandate to privatise marriage".
He contended that the institution of marriage would only be weakened by a redefinition allowing gay couples to marry.
"Civil partnerships will eventually have to be opened to heterosexual couples, individual rights claims will inevitably erode most of the so-called protections for religious groups, and multiple forms of marriage will begin to appear thereby undermining the institution itself," he said.
"What is so crazy about this is that by applying free market principles to marriage the state will also need to enforce a new social orthodoxy by dramatically interfering in family life, religion and society."
Redefining marriage would require "vast and incalculable changes" to local government, the legal system, health, welfare, employment and education, he further warned.
"There is no legal protection for the majority of the ordinary non-religious public who insist that marriage is only between a man and a woman.
"Equality is not the same as uniformity, and the debates around adultery and consummation show that if we change the meaning of marriage it will no longer be marriage," he said.
The Catholic Church said it would continue to support marriage as a lifelong commitment between a man and a woman.
In a statement on behalf of the Church, Archbishop Peter Smith said there were "profound problems" with a change to the law.
"Marriage is rooted in the complementarity of man and woman. For these reasons the Church opposes the Government's Bill to redefine marriage," he said.
"Despite claims by supporters of the Bill that the central issue is one of equality, the Bill actually seeks to redefine marriage and will consequences for society at large."
Christian Concern said a change to the law was "needless and reckless".
"This is a sad day for Britain, when our elected representatives are prepared to overturn centuries of legislation on marriage," said chief executive Andrea Minichiello Williams.
"The essence off marriage has always been the union of one man and one woman.
"The vote today not only has the potential to overturn historic legislation but also centuries of a common understanding of marriage. No longer will a common understanding exist.
"There are many people – Christians and non-Christians – who will never accept that marriage can ever be anything other than between a man and a woman."