Canada's Supreme Court is deliberating after two days of hearings on whether gay marriage is constitutional.
The majority of Canadians are in favour of same-sex marriage
On Thursday, the province of Alberta and religious groups argued marriage was constitutionally defined as a union between a man and a woman.
Supporters of gay marriage were heard on Wednesday. The court is expected to deliver its non-binding legal opinion on the matter in several months.
Gay marriages are legal in only half of Canadian provinces and territories.
Alberta, whose Premier Ralph Klein is an avowed opponent of gay marriage, was the only province to oppose the Ottawa government in court.
The Supreme Court judges challenged several of the arguments put forward by opponents of same-sex unions on Thursday.
In one exchange, a group Focus on the Family argued marriage should be reserved for heterosexual couples because they alone could procreate.
"To reduce the whole thing to procreation seems to be an oversimplification," Justice Ian Binnie said.
And to the argument that heterosexual marriage was the "ideal", Justice John Major said: "The ideal you speak of becomes more and more idealistic, doesn't it?"
Opponents of the draft law also argue that it infringes on the right of churches to refuse to perform same-sex marriages.
On Wednesday, government lawyers opened the landmark case arguing that same-sex marriage should be supported as a matter of justice.
In the run-up to the hearing, few advocates expected the court to rule against the government's plan to make gay marriage legal across the country.
As well as the attorney general, gay marriage is supported by the Canadian Human Rights Commission, the United Church of Canada representing Presbyterian and Methodist congregations, and a coalition of liberal rabbis.
"We are very confident that the Supreme Court will
confirm what many judges have said across the country,"
Laurie Arron of the gay advocacy group, Egale Canada, told the Associated Press.
The government has promised a free vote on the issue in parliament after the ruling.
Canada would join the Netherlands and Belgium in legalising same-sex marriages.
Last month, a court in the Canadian province of Ontario approved what is thought to be the world's first gay divorce.