Sydney Archbishop Anthony Fisher OP has urged parishioners in a Sunday congregation at St Mary's Cathedral to maintain clarity about the definition of "real marriage", ABC News reports.
During his homily, the Archbishop argued the only kind of relationship the state should have an interest in recognising and regulating was heterosexual marriage.
"Governments should, in general, keep out of the friendship business and out of the bedroom," Archbishop Fisher said.
He spoke about heterosexual marriage being primarily focused around the importance of children.
"The only kind of friendship the state has a proper interest in recognising and regulating is heterosexual marriage, because that's what leads to children — new citizens — and gives them the best start in life," he said.
Archbishop Fisher also expressed his concern for religious freedoms.
"In a culture which for all its putative open-mindedness is less and less tolerant of Christianity, how will we ensure in the years ahead that people in parishes, schools and other institutions are free to speak and practice their beliefs?" he said.
"How will we maintain a sense of who we are and what matters most to us when some others barely tolerate us or even vilify and bully us?
"If overseas experience is anything to go by, it will be very hard to speak up for real marriage anymore in schools, at work, socially."
He said he feared some Catholics might face discrimination for voting no.
"Traditional believers will be vulnerable to discrimination suits and other kinds of bullying for their beliefs," he said. "Some may lose their jobs, promotions, businesses, political careers."
It's the first time the Archbishop has centred a Sunday homily on the same-sex marriage debate. Last month, he wrote a letter to principals of New South Wales Catholic schools encouraging them to vote no.
Archbishop Fisher's homily (Sydney Archdiocese)
Sydney Archbishop Anthony Fisher preaches 'no' vote in Mass at St Mary's Cathedral (Sydney Morning Herald)
Fisher preaches No vote (The Weekend Australian)