Stéphane Dion said his decision to allow Liberal MPs to vote their conscience will prevent Stephen Harper from saying 'Dion prevented his MPs from voting freely.' ((CBC))

Opposition MPs demanded Wednesday to know why the Conservative government insists on reopening the controversial debate over same-sex marriage.

The demand came as MPs debated a Conservative motion to introduce legislation to restore the traditional definition of marriage while respecting existing same-sex marriages.

"What is the crisis that the government is responding to?" NDP member Bill Siksay asked at the start of the debate in the House of Commons.

"Is there any documentation showing there is a crisis in marriage?"

Conservative House leader Rob Nicholsonwas quick to contradict him, saying he made the same argument when the Liberal government moved to legalize same-sex marriage across Canadain 2005.

"I raised that point exactly one year ago," Nicholson said. "What was the rush to change the traditional definition of marriage?"

The Conservative motionis not expected to pass on Thursday. The Bloc and NDP are telling their MPs to vote against it.

Liberal MPs allowed to vote freely

Liberal Leader Stéphane Dion announced Wednesday afternoon, just before the debate began, that he will allow his MPs to vote freely on the motion.

Most are expected to oppose it, just as they did in 2005when the government adopted Bill C-38, which legalized same-sex marriage in Canada.

Only 32 Liberals voted againstC-38, while 95 voted for it.

Like the Liberals, Conservatives will also be allowed to vote freely on Thursday. In 2005, only three Tory MPs supported C-38.

Dion, elected as leader on Saturday,said he wants Liberals to vote as they choose because it will send a strong message to Prime Minister Stephen Harper.

"There will be a free vote to prove that an overwhelming majority of my MPs are not willing to vote with the government on this issue,"Dion said before the debate.

"We want that to be over…. It will be the end of the story."

Before the debate on Wednesday, Tory MP Bev Oda said she voted against C-38 and she plans to vote for the Conservative motion on Thursday.

"I will be consistent with theissue," she said.

'I don't think there's any harm'

Conservative MP James Moore is voting against reopening the issue. He said he would only vote for the motion if he felt same-sex marriage was causing public harm.

"And I don't think there's any harm in allowing two people [of the same sex] who want to live in a loving, monogamous relationship … to do so," he said before the debate.

Dion said if there were a vote in the future to change the same-sex law, he would tell his MPs to vote along party lines. That vote should not arise, he added, provided the current motion about revisiting the law is defeated Thursday.

Thursday's vote stems from a promise Harper made leading up to the January 2006 election. He told voters he would hold a free vote on the issue.