Religious believers would be vulnerable to discrimination suits and some could lose their jobs, promotions, businesses and political careers if same-sex marriage is legalised, the Catholic Archbishop of Sydney says.

Anthony Fisher supported the "no" vote in the same-sex marriage postal survey during his homily at Mass on Sunday, telling worshippers the government should "keep out of the friendship business and out of the bedroom".

Anthony Fisher delivers his homily at St Mary's Cathedral. Credit:AAP

"The state has no business telling us who we should love and how, sexually or otherwise," he told the St Mary's Cathedral congregation.

"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."