The love of same-sex couples is "like the love of friends" and they should not be allowed to marry, one of the country's most senior Catholics says.
Archbishop of Brisbane Mark Coleridge said those participating in the same-sex marriage postal survey should consider that same-sex couples were different.
"That love is like the love of friends," he said.
"It is love and it is valuable but it's not and it can't be the kind of love that we call marriage."
Archbishop Coleridge said there had always been discrimination towards some forms of marriage.
"Parents can't marry their children, children can't marry their parents," Archbishop Coleridge said.
"Sibling marrying sibling has always been ruled out.
"People underage have been disqualified from marrying but so too people of the same sex."
He said he was not speaking simply from a religious point of view and said language had become "slippery" in the same-sex marriage debate.
"I am concerned to clarify the language and I don't think it's enough to say that 'love is love'," he said.
"There's only one form of love, the many forms of love that we call marriage and that is between a man and a woman, supposedly lifelong and supposedly open to children."
Archbishop Coleridge said while it was his responsibility to equip fellow Catholics with knowledge about the debate, he did not want to give a "hard time" to people in same-sex unions.
"Their dignity has to be respected at every turn," he said.
The Archbishop said he understood not every Catholic would agree with him.
"There are Catholics who will vote yes and Catholics who will vote no," he said.