SUPPORTERS of gay marriage have lashed out at Catholic Church campaign urging parishioners to oppose it.
Church leaders will urge Victoria's million-plus Catholics to campaign against gay marriage, saying it would undermine family life and damage society.
Catholic Archbishop of Melbourne Denis Hart and five other bishops will send out 80,000 letters this weekend asking parishioners to take a stand against proposed new laws and influence politicians' vote.
One bishop said the push was about protecting traditional marriage, and while today's discussion was on same-sex laws, "next it might be polygamy".
The bishops argue the foundation of marriage is the sexual difference between men and women and the "potential for new life".
"Without this there would be no human beings and no future," the letter says.
"Bringing new human life into the world is founded on the loving union in difference of male and female. Children are best nurtured by a mother and father."
Two Lower House MPs - Labor's Stephen Jones and Green Adam Bandt - have put forward amendments to the Marriage Act.
They argue that any couples should be able to marry, regardless of their sexual orientation.
The Australian Greens today attacked the church's stance, urging supporters of human rights to stand up for love and show they were not marriage sceptics.
This was "unlike some church leaders who think the institution of marriage is too fragile to handle further changes," Senator Sarah Hanson-Young said.
Society had not collapsed in countries such as Canada and Spain where marriage equality had been achieved, she said.
"And it won't happen once Australia moves with the times."
Senator Hanson-Young said for most Australians marriage now was a civil, public celebration of the love between two consenting adults.
Marriage equality supporters have described the church's campaign as "alarmist" and rejected claims gay marriage would undermine family life or damage society.
"Families and societies are only strengthened when couples are allowed to commit to each other through marriage," national convenor of Australian Marriage Equality Alex Greenwich said.
"So to hear Archbishop Hart discouraging any recognition of this commitment is extraordinary and heartless."
Former NSW premier Kristina Keneally, a devout Catholic, said people of her faith should look at a range of information sources to formulate their views.
"I've come to a position, with a fully-formed conscience, that I support gay marriage," she told ABC Television.
"I would encourage all Catholics to apply critical thinking to this issue."
Ms Keneally said the teachings of the church were not infallible although it was important people take heed of what their parish priest or bishop was saying.
"But it's equally important for them to consider how they in good conscience must act."
An inquiry into the Bill allows Australians to answer questions online, including on whether they agree that same-sex marriage should be legalised.
About 54 per cent of 65,000 Australians who have completed the online survey supported the change.
The Catholic bishops are asking their parishioners to vote in that online survey.
Sale Bishop Christopher Prowse, one of the signatories to the letter, said the church's directive was about highlighting the "true meaning of marriage".
"At this stage (the debate) is about same-sex marriage, but next it might be polygamy," Bishop Prowse said.
The church's push was not about a lack of respect for people who identify as gay and lesbian, who should be treated with respect and dignity, he said.
But the church believed politicians had no right to change the definition of marriage because it pre-dated laws.
"We have asked Catholics to seriously reflect and pray about the ramifications ... of legislation which completely redefines marriage," he said.
