Sydney's Archbishop Anthony Fisher has urged parishioners in a Sunday congregation at St Mary's Cathedral in the city's CBD to maintain clarity about the definition of "real marriage".

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.

Congregation gives mixed response

Outside St Mary's, just across from Hyde Park, the ABC spoke to some of the congregation who had heard Archbishop Fisher's homily.

Liz Helson outside St Mary's Cathedral after Archbishop Anthony Fisher's homily on same-sex marriage. ( ABC News: Lily Mayers )

Liz Helson, from Brisbane, said the speech was "absolutely brilliant".

"I think it actually defined marriage as opposed to love," Ms Helson said.

"I'm all for homosexuals and I'm all for gay love, but the definition there was for a husband and wife to be able to procreate."

Catholic nun Sister Helen did not agree with the Archbishop but said she respected his opinion.

"I think the church may advise and direct people but I really don't think the church should dogmatically rule in things like that," Sister Helen said.

"I have friends in a homosexual relationship and they are committed to each other and love each other.

"I think God is love and I am okay with them committing to each other."

Jackson Jois said he agreed with the Archbishop's views on same-sex marriage.

"It's something we can share thoughts about but ultimately it's the decision of the Government," Mr Jois said.

Tom Jamieson and Hannah Harrison did not agree with Archbishop Fisher's stance. ( ABC News: Lily Mayers )

Hannah Harrison said she thought we should "accept everyone for who they are".

"I think [Archbishop Fisher] should really think about the feelings of others and how it's going to affect their lives," she said.

"Marriage between two people doesn't govern the fact that it's between a man and a woman, it governs the fact that two people love each other," her friend Tom Jamieson said.

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.