MPs debated late into the night Wednesday on whether Parliament should restore the traditional definition of marriage.

A free vote is planned for Thursday. Last year's vote passed 158-133 in favour of allowing same-sex marriage.

Earlier in the day, Conservatives defended the debate, arguing the issue is divisive and still warrants discussion -- although, Prime Minister Stephen Harper and several of his ministers were absent.

"Some members may question why it's necessary to engage this House on this matter," Rob Nicholson, the government's house leader and democratic reform minister, told the House of Commons on Tuesday.

"The debate surrounding Bill C-38 generated a significant amount of controversy. It was a divisive debate, both in the House and among Canadians as a whole, and that debate continues."

Nicholson said given the importance of marriage to society, Parliament needs to revisit the issue with a truly free vote.

The actual motion reads as follows: "That this House call on the government to introduce legislation to restore the traditional definition of marriage without affecting civil unions and while respecting existing same-sex marriages."

He noted passing this motion won't change the law passed by Parliament in June 2005 that allowed for same-sex marriages.

However, if it passes, the government will introduce legislation that will define marriage as the union of one man and one woman to the exclusion of all others, Nicholson said.

Bill Siksay, an NDP MP, asked what the purpose was in restarting the debate when the last Parliament dealt with the issue.

Nicholson said in that earlier debate, he asked what was the rush to change the traditional definition "which has been around for 2,000 years."

Liberal MP Bill Graham said the government knows that eight provincial courts of appeal had ruled the traditional definition of civil marriage was unconstitutional.

The courts found that the traditional definition violated the equality rights of gays and lesbians under the Charter of Rights and Freedoms.

While the courts can allow an unconstitutional law to stand under certain circumstances, no appeal court did so.

Graham said the only way for the government to restore the traditional definition is to use the "not withstanding" clause in the Charter, which allows governments to over-ride the courts.

"My understanding is that the government has rightly said it will not use the not-withstanding clause. I applaud it for that, and I assume it will stick to that," he said.

In any event, Graham said this whole issue is in a "grey zone" because the debate "is about holding a debate," rather than on actual legislation.

Globe and Mail columnist Jeffrey Simpson wrote Wednesday that Wednesday's debate was "legal nonsense" because the only way to change the law is to use the not-withstanding clause.

He also suspected the Harper government, which made a campaign promise to re-open debate, would secretly like to see the motion defeated so that the issue will go away.

A Strategic Counsel poll released Sunday indicates that 58 per cent of Canadians say they would vote to keep the current same-sex marriage law, while 36 per cent would vote to scrap it.

Religious leader Charles McVety, who heads the Canada Family Action Coalition, is among those who have urged the Conservatives to re-open the debate.

"Children's rights have been stripped away," he told reporters. "Nature gives the rights to a child to have two parents, a mother and a father."

Free vote

Liberal MPs will be permitted to vote freely on a Conservative government motion to reopen the same-sex marriage debate, party leader Stephane Dion decided after a caucus meeting on Wednesday.

The newly minted leader is not in favour of reopening the contentious debate, but he is concerned that if he were to impose party discipline, Prime Minister Stephen Harper and his Tories would gain political leverage.

"We would have to have a party vote because we have to protect the Charter of Rights. We have to reply with a tactic that will destroy his tactic," Dion told reporters in Ottawa.

"The only way to prove that is with a free vote to prove a very large majority of Liberals are against the prime minister's actions."

Dion added that a free vote will allow Liberals to really demonstrate that the issue is closed, and that it would prevent the issue from being reopened if the Tories were to say he "whipped his MPs," meaning he had pressured them to vote a certain way.

Liberal MP John McKay, who is opposed to gay marriage, told Canada AM Wednesday that a "free and open" vote is essential to bringing closure to the issue.

"One of the complaints for people like myself, who supported the traditional definition of marriage, is that previous votes have not been free and in both instances the cabinets were whipped and that skewered the vote in the house," said McKay.

"If you want to actually have closure to this issue you make it free and open and let the MPs speak."

When the Liberal minority government passed same-sex marriage legislation in 2005, 32 Liberal MPs voted against it. Cabinet ministers and parliamentary secretaries had to vote for it.

As a result, more than 12,000 gay and lesbian couple across Canada have married.

While the focus has been on the Liberals, the NDP and Bloc Quebecois will impose party discipline in an effort not to reopen the debate.

The two parties hold 80 seats and the Liberals have 102. There are two Independents.

There are 124 members of the Conservative caucus.

Some Conservatives expected to vote against the motion include cabinet ministers John Baird, Jim Prentice, Loyola Hearn and David Emerson.

With files from The Canadian Press