Pope Benedict, speaking out on a topic that Italy's incoming centre-left government will likely have to confront, on Thursday condemned gay marriage and legal recognition of unwed couples.
The 79-year-old German Pope immediately came under fire from some leftists who accused him of trying to write the country's political agenda.
The Pope, speaking to a conference on marriage and the family, reaffirmed the Church's position that marriage had to be a union between a man and a woman and open to procreation.
"Only the rock of total and irrevocable love between a man and a woman is capable of being the foundation of building a society that becomes a home for all mankind," he said.
He told the group that marriage was between a man and a woman "who are open to the transmission of life and thus cooperate with God in the generation of new human beings".
The coalition of incoming Prime Minister Romano Prodi promises some form of recognition for unmarried couples but has stopped short of openly supporting gay marriage as part of its programme.
However, some coalition parties back greater rights for homosexuals, including marriage, and the issue is widely expected to surface sooner or later after the government is sworn in next week.
Franco Grillini, a leftist parliamentarian who is openly gay, accused the Pope of trying "to write a political agenda" and of "ignoring the rights of million of Italians who live together".
Vladimir Luxuria, Europe's first "transgender" lawmaker and a member of Prodi's coalition, went further in criticising the Pope, saying it was the duty of a lay state to "recognise and regulate" homosexual unions.
Italy's Catholic Church has already served notice to the centre left that it will fight any move to recognise civil partnership for unwed heterosexual couples and gay couples.