5 February 2013 Last updated at 18:01 GMT
Gay marriage an important step forward, says PM
Same-sex marriage is "an important step forward" and will "make our society stronger", David Cameron has said.
The prime minister's intervention came shortly before MPs were due to vote on plans to legalise gay marriage in England and Wales.
A number of Conservative MPs have spoken out against the plans - one calling the idea "Orwellian".
But Mr Cameron said he backed marriage for all couples because it was right and it promoted commitment.
"Today is an important day. I am a strong believer in marriage. It helps people commit to each other and I think it is right that gay people should be able to get married too," he said.
"This is, yes, about equality. But it is also about making our society stronger. I know there are strong views on both side of the argument - I accept that. But I think this is an important step forward for our country."
The Marriage (Same Sex Couples) Bill would enable same-sex couples to get married in both civil and religious ceremonies, where a religious institution had formally consented, in England and Wales.
It would also allow couples who had previously entered into civil partnerships to convert their relationship into a marriage.Adultery
Minister for Women and Equalities Maria Miller told MPs: "What marriage offers us all is a lifelong partner to share our journey; a loving stable relationship to strengthen us and a mutual support throughout our lives.
"I believe this is something that should be embraced by more couples. The depth of feeling, love and commitment is no different between same-sex couples than opposite-sex couples."
Mrs Miller argued that marriage had evolved over time and rejected the claim that there was no need for same-sex marriages because same-sex couples can already have a civil partnership.
"A legal partnership is not perceived in the same way and does not have the same promises of responsibility and commitment as marriage," she said.
"All couples who enter a lifelong commitment together should be able to call it marriage."
She acknowledged the concerns of religious groups about the plans but said there need not be a choice "between religious belief and fairness for same sex couples".
But some MPs raised concerns that, by changing marriage from being solely between a man and a woman, fundamental parts of marriage, such as the ability to divorce on the grounds of adultery or failure to consummate the marriage, would not apply to same-sex marriages.'Orwellian'
This is because current legal definitions of adultery and consummation are based on sexual contact between a man and a woman.
"There is absolutely no doubt that once marriage is re-defined in this very fundamental way, a whole number of new legal questions will arise, and no one can be quite sure what the outcome will be," warned Sir Tony Baldry, a Conservative MP and the Church of England's representative in the Commons.
"The government believes that this is a risk worth taking; the Church of England believes that it is not."
"Marriage is the union between a man and a woman, has been historically, remains so. It is Alice in Wonderland territory, Orwellian almost, for any government of any political persuasion to seek to come along and try to re-write the lexicon. It will not do", said Sir Roger Gale, the Conservative MP for Thanet North.
"There is a way forward. It's been suggested, but it's been ignored," he added.
"I don't subscribe to it myself, but I recognise the merit in the argument, and that is this: if the government is serious about this, take it away, abolish the Civil Partnerships Bill, abolish civil marriage, and create a Civil Union Bill that applies to all people irrespective of the sexuality, or their relationships, and that means brothers and brothers, and sisters and sisters, and brothers and sisters as well.
"That would be a way forward. This is not."'Right side of history'
Reports suggest upwards of 120 Conservative MPs could vote against the government's plans, including some cabinet ministers.
MPs will have a free vote on the bill, meaning they will not be ordered to vote for or against by party whips.
The legislation is expected to pass through the Commons with most on the Labour and Liberal Democrat benches supporting it.
For Labour, Yvette Cooper called on MPs to back the proposals, saying: "Couples who love each other should be able to get married regardless of their gender and their sexuality."
She said she wanted to see same-sex relationships "celebrated and valued by the state in the same way as everyone else".
Ms Cooper argued that the bill would keep the institution of marriage "inclusive and in touch for the next generation".
"Let's celebrate not discriminate, and let's be on the right side of history, and vote for this bill today," she urged MPs.
On Tuesday, Chancellor George Osborne, Foreign Secretary William Hague and Home Secretary Theresa May wrote to the Daily Telegraph in an attempt to persuade Tory MPs to back the plan.
They wrote: "Marriage has evolved over time. We believe that opening it up to same-sex couples will strengthen, not weaken, the institution."
The government believes that the wording of the bill will ensure that the Church of England and the Church in Wales will not face any legal challenges to their strong stated opposition to holding same-sex marriages.
All religious organisations will be able to opt in to holding ceremonies - but the Church of England and the Church in Wales would first need to agree to change canon law.
5 February 2013 Last updated at 08:59 GMT
Gay marriage: Party leaders hail vote
MPs have backed a bill to allow same-sex couples to get married in England and Wales, while the French National Assembly last week approved the most important article of a bill to legalise same-sex marriage.
But where in the world can same-sex couples already get married?
Just after midnight on 1 April 2001, four couples - Anne-Marie Thus and Helene Faasen, and three male couples - were married by the mayor of Amsterdam, Job Cohen, in the first legal gay marriage ceremony in the world.
"We are so ordinary, if you saw us on the street you'd just walk right past us," said Anne-Marie Thus of the fuss over the televised City Hall ceremony.
"The only thing that's going to take some getting used to is calling her my spouse."
Denmark was the first country to introduce civil partnerships for same-sex couples, in 1989, but it stopped short of allowing church weddings.
Countries including Norway, Sweden and Iceland followed suit in allowing partnerships offering many - but not all - of the rights and obligations of marriage.
But it was left to the Netherlands to lead the way in allowing gay marriage, which included granting same-sex couples the right to adopt children.
It was a move welcomed by international gay rights groups as a huge step forward.Outright bans
A few weeks after neighbours Belgium followed the Netherlands' example in June 2003, the Vatican - in an attempt to stop further legislation - launched a global campaign against gay marriage.
In a strongly-worded 12-page document, Pope John Paul II's chief theological adviser Cardinal Joseph Ratzinger - who would go on to become Pope Benedict XVI - warned that homosexual unions were immoral, unnatural and harmful.
Two years later, despite a 600,000-strong petition organised by a Catholic group and a rally in Madrid opposing it, same-sex marriage was introduced in Spain.
Emilio Menendez and his American partner of 30 years, Carlos Baturin German, became the first gay couple to tie the knot in Spain, at a ceremony in Tres Cantos, outside Madrid, on 11 July 2005.
Days later, Canada - where same-sex marriage had already been permitted in most provinces since 2003 - became the fourth country to introduce national legislation.
With the US slow to follow - a federal law still prevents US recognition of gay marriage and many states have enacted outright bans - thousands of gay Americans have visited Canada to get married since 2003.
Same-sex marriage is now allowed in nine American states as well as the District of Columbia.Court ruling
South Africa, in November 2006, became the first African country to bring in marriage for gay couples - despite homosexuality remaining taboo in large parts of the continent.
That followed a 2004 Supreme Court of Appeal ruling - brought by lesbian couple Marie Fourie and Cecilia Bonthuys - that existing marriage laws discriminated against same-sex couples.
In January 2009, Norway became the sixth country to introduce gay marriage followed, in May of the the same year, by Sweden, while a further three countries followed suit in 2010.
Divorced mothers Teresa Pires and Helena Paixao became the first in Portugal, in June 2010 - a month after the law they had campaigned for came into effect - and hailed it as a "great victory, a dream come true".
The socialist government in the mainly Catholic country faced fierce opposition from campaigners who ultimately failed to get enough support for a referendum.
Later that month, Iceland's prime minister Johanna Sigurdardottir married her partner, writer Jonina Leosdottir, on the day the country's gay marriage law came into force.
In July 2010, meanwhile, Argentina became the first country in Latin America to legalise gay marriage.
Up until then, Mexico City had been the only place in the region where same-sex marriage was allowed.
And, in June last year, Denmark became the 11th country to approve same-sex marriage - 23 years after it became the first country in the world to recognise gay civil partnerships.