Pope Benedict today restated the Catholic Church's position that only unions between a man and a woman are moral.
The pontiff was speaking a day after a California court ruled in favour of same-sex marriage, although he made no mention of the that decision in his speech to family groups from throughout Europe.
But he 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 US state.
The court found laws limiting marriage to heterosexual couples are at odds with rights guaranteed by the state's constitution.
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, the Catholic Church successfully campaigned against an Italian law proposed by the previous centre-left government that would have given more rights to gay and unmarried couples.
The Catholic Church teaches that homosexuality is not sinful but homosexual acts are, and it is opposed to gays being allowed to adopt children.