The government has launched a 12-week consultation on allowing gay couples in England and Wales to marry.
The proposal is being fiercely opposed by some senior church figures, as well as a number of Conservative MPs.
Civil partnerships, introduced in 2005, already give gay couples similar legal rights to married couples.
But the government wants them to be legally allowed to make vows and declare they are married before the next general election, due in 2015.
TheHome Office's consultation paperproposes:
- to allow same-sex couples to marry in a register office or other civil ceremony
- to retain civil partnerships for same-sex couples and allow couples already in a civil partnership to convert it into a marriage
- to allow people to stay married and legally change their gender
- to maintain the legal ban on same-sex couples marrying in a religious service
Liberal Democrat Equalities Minister Lynne Featherstone said: "We're not looking at changing religious marriage, even for those that might wish to do it.
"I understand the liberal Jews, the Quakers and some unitarian churches would like it, but that's not in the sight of this consultation."
Labour welcomed the proposals but said they did not go far enough.
Shadow home secretary Yvette Cooper said: "Religious marriages are a matter for each church and denomination, not for the government. But equally, the government should go further than they currently plan.
"Churches who want to celebrate gay marriage [should have] the chance to do so."
Gay rights campaigner Peter Tatchell also welcomed the government's commitment to legalise same-sex civil marriages but said he was unhappy about the continued ban on religious same-sex marriages.
"This is not only homophobic but also an attack on religious freedom. While no religious body should be forced to conduct same-sex marriages, those that want to conduct them should be free to do so."
Mike Judge, from the campaign group Coalition for Marriage, said: "Marriage is so much part of everyday life. If we change its meaning in law, it will have a knock-on effect in everyday life."
He pointed to Spain which has changed birth certificates to say 'progenitor A' and 'progenitor B' instead of mother and father since same-sex marriage was legalised there.
The Home Office is also asking individuals and organisations to give their views on the proposals for England and Wales inan online survey.
The Liberal Democrats have long campaigned for reform of the marriage laws, arguing that they are outdated and discriminate against same-sex couples.
While in opposition, Prime Minister David Cameron backed a move to consider allowing civil partnerships to be classified as marriage, as part of his modernising drive in theConservative Party's Contract For Equalities, published in May 2010.
However, some Conservative MPs are uncomfortable with the move, arguing it will undermine the traditional idea of the family.
When legislation comes before the Commons, Tory MPs are expected to be offered a free vote to avoid an embarrassing backbench revolt.
'Shame' on UK
Earlier this month, during Commons questions about the consultation, Conservative backbencher Peter Bone said: "Wouldn't it just be very simple to write back and say: 'Marriage is between a man and a woman so this is completely nuts'?"
Meanwhile, senior members of the clergy have complained that politicians should not be allowed to redefine marriage.
Cardinal Keith O'Brien, the leader of the Roman Catholic Church in Scotland, said the "grotesque" plans would "shame the United Kingdom in the eyes of the world" if implemented.
A week later, Roman Catholic congregations across England and Wales were read a letter from the Church's two most senior archbishops saying the change would reduce the significance of marriage and it was the duty of all Roman Catholics to make sure it did not happen.
The leader of the Church of England, Archbishop of Canterbury Dr Rowan Williams, has said the law should not be used as a tool to bring about social changes such as gay marriage.
Ben Summerskill, chief executive of gay rights charity Stonewall, has said the issue was neither about religious freedom nor party politics.
"Ultimately it's about the freedom of a small group of people to be treated in exactly the same way as everyone else," he said.
The Scottish government held its own consultation process and received more than 50,000 responses.
A number of other countries already allow same-sex couples to marry, including Spain, Canada, Argentina, the Netherlands, Portugal, South Africa, Sweden and Belgium.