Voting on gay marriage is not "an imposition on religion or churches" and should not be left to MPs' consciences, Nick Clegg said on Sunday.
The Deputy Prime Minister and Liberal Democrat leader said his MPs would back legislation legalising same-sex marriages - unlike some of their coalition partners.
Tory MPs will be allowed to vote either for or against same sex marriage, to avoid a rebellion from Tory MPs and ministers opposed to the issue.
Mr Clegg differentiated himself from his coalition partner saying it was up to Prime Minister David Cameron whether he forced Conservative MPs to support the measure, but vowed Lib Dems would "honour what we have said as a party".
"We are not asking any person with religious convictions to sacrifice anything.
"We are simply saying those who want to show a lifelong commitment to each other should be able to do so."
Commons Leader Sir George Young last week confirmed MPs would be offered a free vote, saying: "Along with other issues that involve matters of conscience, it seems to me perfectly proper that this should be subject to a free vote on this side of the House and that is what we plan to do."
But Mr Clegg told BBC1's Andrew Marr Show: "In the same way the civil partnerships legislation which was introduced under Labour was a whipped vote, I personally don't think this is something which should be subject to a great free for all because we are not asking people to make a decision of conscience."
He added: "If this was an issue that somehow the government was proposing something that would somehow be an imposition on religion or the churches, then of course that would be a matter of conscience. We are not."
Critics have questioned Mr Cameron's commitment to the reform after he conceded to a free vote to quash an expected a rebellion among backbench Tories.
Conservative Cabinet minister Northern Ireland Secretary Owen Paterson also declared his opposition to same-sex marriage in the most high-level challenge to Mr Cameron so far on the issue, while defence minister Gerald Howarth is also against the move.
Tory critics view the proposal as a Liberal Democrat policy distracting the government from bigger challenges.
Meanwhile Sinitta is the latest celebrity to back a campaign supporting global equal marriage rights for gay couples, Out4Marriage.
The singer said: "I believe in love and I also believe in marriage. I've been married and I hope one day to marry again.
"Most of all I believe in all people being able to celebrate their love in marriage; gay people, straight people, black people, white people - all people. That's why I'm Out4Marriage, are you?"
Sinitta joins entrepreneur Sir Richard Branson, girl group The Saturdays, comedian David Walliams, shadow home secretary Yvette Cooper, actor and writer Stephen Fry and gay rights campaigner Peter Tatchell, who have all given their support to the campaign
Carl Froch knocks out Lucian Bute during their IBF World Super Middleweight Title bout at Nottingham Capital FM Arena on May 26, 2012 in Nottingham, England. (Photo by Scott Heavey/Getty Images)
A man trying to catch a young wild stallion in Duelmen, Germany, Saturday, May 26, 2012. It is believed that this is the last herd of wild horses on the European continent in this 350 hectare reservation in western Germany, where more than 300 of the indigenous horses stay free and wild without any care under the protection of the Duke of Croy. Only once a year the new born stallions are caught and sold, all mares stay with the herd and guarantee the survive of the endangered species. (AP Photo/Martin Meissner)
Miami Marlins center fielder Chris Coghlan stretches for a triple hit by San Francisco Giants' Ryan Theriot in the eighth inning of a baseball game in Miami, Saturday, May 26, 2012. The Marlins defeated the Giants 5-3. (AP Photo/Lynne Sladky)
An animal rights activist dressed as a cat protests the Ringling Brothers Circus outside the circus venue before the start of a show in Mexico City, Saturday, May 26, 2012. (AP Photo/Marco Ugarte)
A resident watches from atop a building as fire fighters, unseen, spray water after a fire broke out at a garment factory, left, in Gandhi Nagar area of New Delhi, India, Sunday, May 27, 2012. No casualty has been reported so far and the reason for the fire is not known. (AP Photo/Manish Swarup)
A fisheye view with a wide-angle lens shows center court Philippe Chatrier as Jo-Wilfried Tsonga of France and Andrey Kuznetsov of Russia play their first round match at the French Open tennis tournament in Roland Garros stadium in Paris, Sunday May 27, 2012. (AP Photo/Michel Spingler)
Will Power (12), of Australia, drives under Mike Conway, of England, after they made contact in the first turn during IndyCar's Indianapolis 500 auto race at Indianapolis Motor Speedway in Indianapolis, Sunday, May 27, 2012. (AP Photo/Tom Hemmer)
The Phoenix fireboat sprays plumes of water as part of the Golden Gate Bridge's 75th anniversary celebration on Sunday, May 27, 2012, in San Francisco. The commemoration included a vintage boat parade and a fireworks display slated for the evening. (AP Photo/Noah Berger)
Jessica Ennis of Great Britain competes in the Women's High Jump in the women's heptathlon during the Hypomeeting Gotzis 2012 at the Mosle Stadiom on May 26, 2012 in Gotzis, Austria. (Photo by Ian Walton/Getty Images)