The bizarre warning came after Milan's liberal mayor, Giuliano Pisapia, announced that the city was to begin a register of civil unions.
Although largely symbolic, Mr Pisapia said that his plan would provide some additional legal rights for cohabiting couples who were unable to marry.
"The establishment of a register of civil unions is aimed at recognising and protecting the rights of many couples in Milan and the rest of the country, couples that have been waiting too long for legal recognition," he said.
Mr Pisapia noted the key ruling earlier this year by the Supreme Court of Cassation that said same-sex couples had a "right to a family life" – and by implication, the same benefits and rights as straight couples.
Italy's record on improving the legal rights of its gay, lesbian and transgender citizens is probably the worst in western Europe.
Catholic parliamentarians on the left and right have blocked anti-hate legislation aimed at combating the growing number of violent attacks against lesbians and gays.
It is the only country in western Europe to allow neither civil partnerships nor marriage for gay couples.
The amiable, scruffy Mayor Pisapia – who cuts an unlikely figure in Italy's hard-nosed financial and fashion capital – promised to help rectify the situation when he campaigned for office two years ago.
But his announcement nonetheless provoked a furious response from the Catholic Church.
Alfonso Colzani, the spokesman on family issues for the powerful Milan diocese said: "There's the risk that giving equal status to families based on marriage with those founded on civil unions will legitimise polygamy."
This was, he said, because people in civil partnerships would be freer to have sexual relations with other people.
He added: "Introducing "a communal register of civil unions is an ineffective initiative – and maybe only a PR exercise. Instead it is the family that needs support in this time of hardship. The concept of marriage is a precise one and not to be confused with homosexual unions."
The Milan diocese, Italy's biggest and richest, is headed by Cardinal Angelo Scola, who has strong links with Communion and Liberation, the ultra-conservative catholic political lobbying group. Cardinal Scola was selected for the post of Milan's archbishop last year by Pope Benedict.
The pontiff himself declared in 2010 that gay marriage was a bigger threat to the human race than disease, famine and terrorism.
Last week, in an attempt to scupper Mr Pisapia's initiative, Il Giornale, the reactionary newspaper owned by the Berlusconi family, published an open letter opposing the mayor's plans written by a group claiming to represent the parents of gays and lesbians.
Within hours, the national gay rights group, Arcigay, had pointed out that the association pursued questionable policies such as advocating the "cure" of gays through prayer.
In a statement, Arcigay said opposition to simple civil unions shown by the church, right-wing newspapers and politicians across the political spectrum "showed an incredible
concentration of homophobia and prejudice".
In the past two weeks Italy's main centre-left political grouping, the Democratic Party, has seen internal battles over the issue of civil partnerships with party president, Rosy Bindi, accused of blocking moves for the party to assume a pro-civil partnership position.
In response, Ms Bindi denied she was homophobic and attempted to portray Catholics as victims, calling for their views to be heard.