Pope Benedict, speaking a day after a California court ruled in favour of same-sex marriage, firmly restated on Friday the Roman Catholic Church's position that only unions between a man and a woman are moral.
Benedict made no mention of the California decision in his speech to family groups from throughout Europe, but stressed the Church's position several times.
"The union of love, based on matrimony between a man and a woman, which makes up the family, represents a good for all society that can not be substituted by, confused with, or compared to other types of unions," he said.
The pope also spoke of the inalienable rights of the traditional family, "founded on matrimony between a man and a woman, to be the natural cradle of human life".
On Thursday, the California Supreme Court overturned a ban on same-sex marriages in a major victory for gay rights advocates that will allow homosexual couples to marry in the most populous U.S. state.
Last year, Italy's powerful Catholic Church successfully campaigned against a law proposed by the previous centre-left government that would have given more rights to gay and unmarried couples.
The Roman Catholic Church teaches that homosexuality is not sinful but homosexual acts are, and is opposed to gays being allowed to adopt children.
The California court found laws limiting marriage to heterosexual couples are at odds with rights guaranteed by the state's constitution.
U.S. President George W. Bush, who is opposed to gay marriage, prayed "for the family" with the pope at the White House last month during the pontiff's visit there.
Last year, Cardinal Angelo Bagnasco, the head of the Italian Bishops Conference, made headlines with comments that critics said equated homosexuality with incest and paedophilia.
After he made the comments - which Bagnasco said were misunderstood - graffiti reading "Shame" and "Watch Out Bagnasco" appeared on the door of the cathedral in northern Genoa, where Bagnasco is archbishop.
The pope, who backed Bagnasco, will visit Genoa his weekend.
Opponents of gay marriage in the United States vowed to contest the ruling with a state-wide ballot measure for a constitutional amendment to ban gay marriages.
