Two prestigious Catholic schools have spoken out in defence of gay marriage, arguing the sacrament must evolve with the times.
The rectors of Melbourne's Xavier College and Sydney's Saint Ignatius' College, whose alumni include Opposition Leader Bill Shorten and former prime minister Tony Abbott respectively, have written to parents and staff arguing the Catholic Church's understanding of marriage stretches beyond procreation.
It comes days after Melbourne Archbishop Denis Hart urged Catholics to vote against change in the upcoming postal survey on gay marriage for the "health and future" of society.
Archbishop Hart believes same sex marriage could weaken society.
"I do believe that marriage between a man and woman and family are the bedrock of society and if we follow that through then it could weaken us," he told radio 3AW on Tuesday.
But Xavier College rector, Father Chris Middleton warns there is a "real disconnect" between the church's opposition to gay civil marriage and the attitudes of young people.
"In my experience, there is almost total unanimity amongst the young in favour of same-sex marriage, and arguments against it have almost no impact on them," he writes in the school newsletter.
"Whatever of the postal vote, the Church needs to reflect on why there is such strong support for same-sex marriage among the young.
"They are driven by a strong emotional commitment to equality, and this is surely something to respect and admire."
He said young people understood the destructiveness of homophobia and had an idealistic view of love - "the primary gospel value".
"Any argument against same-sex marriage must respectfully address these core values, or they will fail a basic test of credibility with our young."
Father Ross Jones of Saint Ignatius' College argues same-sex couples want to commit to each other for the same reasons as heterosexual couples, "by reflecting on experience and on what it is to be human, using their God-given reason."
Father Middleton said understandings of marriage within the Catholic Church had evolved with the times, in areas such as arranged marriages, the role of women and divorce.
He also warned the church's ability to speak out against gay marriage had been compromised by allegations of sexual abuse among its clergy.
He said important issues around religious freedom were more likely to be respected if the church was not seen as an "uncompromising enemy" of same-sex marriage.
Meanwhile, the impact a yes vote could have on school children is the focus of television ads from the Coalition for Marriage which began airing on Tuesday.
The ads feature concerned mothers warning a yes vote would mean a tick of approval for "radical" gay sex education in schools.
The group's spokeswoman Sophie York said millions of Australians were concerned about the consequences of a yes vote.
"Australian parents have a right to know how a change in the marriage law will affect what their kids are taught at school," she said.