The Canadian Conference of Catholic Bishops says the federal government has a duty to protect the definition of marriage.
The head of the conference says defining marriage as a union between a man and a woman is not a violation of human rights, but a reasonable distinction.
Most Rev. Jacques Berthelet says the federal government is confusing equality with uniformity.
The federal government has submitted a draft law to the Supreme Court that would change the definition of marriage to include same-sex couples.
The CCCB is calling on Canadians to denounce Ottawa's proposed changes to the legal definition of marriage. The bishops say traditional marriage doesn't discriminate. They argue the legislation would cause irrevocable damage to society.
The bishops say marriage pre-dates both government and the church. They say the state has the duty to protect the traditional definition of marriage to ensure the future of society.
The archbishop of Montreal, Jean-Claude Cardinal Turcotte, says the purpose of marriage is to procreate and he says children need both a mother and a father. He says the proposed law confuses equality with uniformity. And he says it could set a dangerous precedent.
"I give the example of a brother and a sister. They love themselves very much. A father and his girl. He loves her very much. Does that constitute a marriage?"
At least two Catholic priests in Canada have spoken out in support of gay marriage, but the church reacted differently in the two cases. Newfoundland priest Fr. Paul Lundrigan was publicly reprimanded by the archbishop of St. John's.
But when Quebec priest Fr. Raymond Gravel called the Vatican's statement on same-sex marriages "discriminatory, hurtful and offensive," he wasn't reprimanded.
Fr. Gravel says the explanation is that the Catholic Church in Quebec has evolved differently than in the rest of Canada. He says "Quebec Catholics might be closer to the realities of society and more accepting of homosexuals."
Cardinal Turcotte acknowledges the church has to do more to respect gays and lesbians, but he says changing the definition of marriage is not the way.