Same-sex couples do not "qualify" for marriage just as parent and child, or brother and sister, cannot marry each other, one of Australia's most senior Catholics has said.
Archbishop Mark Coleridge of Brisbane made the comparison in an interview with the ABC on Tuesday, and concluded love within same-sex couples was, like "the love of friends", not suitable for marriage.
He appealed to the historical definition of marriage that excludes parent-and-child underage children and siblings, and noted same-sex couples had also traditionally been disqualified from the institution.
"Parents can't marry their children, children can't marry their parents ... sibling marrying sibling has always been ruled out," Archbishop Coleridge said.
Pressed by host Michael Rowland, he conceded the comparison was "not exactly the same" as a same-sex relationship but went on to explain: "That is not to say that they are not equal. It's simply saying that they are not the same and that they don't qualify for what we call marriage.
"There's only one form of love that is marriage, and that is between a man and a woman."
The Archbishop said love between same-sex couples was "like the love of friends ... it is valuable, but it's not, and it can't be, the kind of love that we call marriage".
The phrase "marriage equality" - the preferred term among same-sex marriage advocates - was "a brilliant piece of marketing", Archbishop Coleridge told the ABC, but was ultimately "slippery" and misleading.
"Is it about equality? Well, yes and no," he said. "Every human being is equal, but not all are the same."
Alex Greenwich, chairman of the Equality Campaign, told Sky News the comparison of same-sex couples to incestuous relationships was "offensive" but "nothing new" in the debate over marriage equality.
Co-chair Janine Middleton, a practising Catholic, also reprimanded the archbishop.
"What we would want people to remember is that when they speak, words can inflict real damage, and there's a need to be respectful in the choice of words we use," she said.
Archbishop Coleridge, through a spokesman, declined a follow-up interview with Fairfax Media on Tuesday.
His comparison of same-sex relationships to friendship evoked Catholic Liberal MP Kevin Andrews' observation last month that while he had "an affectionate relationship with my cycling mates", that did not constitute a marriage.
The leadership of the Catholic Church has formed a prominent voice opposing same-sex marriage in the postal survey campaign, with its most senior archbishops in Sydney, Melbourne and Brisbane firmly against a change in the law.
In August, Archbishop Denis Hart of Melbourne, chair of the Australian Catholic Bishops Conference, was among a number of senior Catholics to warn employees of church parishes and schools could face the sack if they entered a same-sex marriage.
Archbishop Antony Fisher of Sydney was an early entrant in the postal survey debate, warning religious freedom was under threat and asking whether Catholic schools would be free to teach their own definition of marriage if same-sex marriage were legalised.
But several polls have showed majority support for same-sex marriage among lay Catholics - more so than other denominations, according to a recent Newgate Research survey, commissioned by The Equality Campaign.
Two of Australia's largest Catholic schools, St Ignatius Riverview in Sydney and Xavier College in Melbourne, have also cautiously sympathised with same-sex marriage in letters to students, parents and staff.
