Pope Benedict on Friday signaled the Vatican was ready to forge alliances with other religions against gay marriage, saying the family was threatened "to its foundations" by attempts to change its "true structure".
The pope's latest denunciation of gay marriage came in a Christmas address to Vatican officials in which he blended religion, philosophy, anthropology and sociology to illustrate the position of the Roman Catholic Church.
The Vatican has gone on the offensive in response to gains for gay marriage in the United States and Europe, using every possible opportunity to denounce it through papal speeches or editorials in its newspaper or on its radio station.
Throwing the full weight of his office behind a study by France's chief rabbi on the effects the legalization of gay marriage would have on children and society, he said:
"There is no denying the crisis that threatens it (the family) to its foundations - especially in the Western world."
The family had to be protected because it was "the authentic setting in which to hand on the blueprint of human existence", he added.
Speaking in the frescoed Clementine Hall of the Vatican's Apostolic Palace, the 85-year-old pope said the family was being threatened by "a false understanding of freedom" and a repudiation of life-long commitment in heterosexual marriage.
"When such commitment is repudiated, the key figures of human existence likewise vanish: father, mother, child - essential elements of the experience of being human are lost," the leader of the world's 1.2 billion Catholics said.
In the speech, one of the most important the pope gives every year, he said people could not "dispute the idea that they have a nature, given by their bodily identity, that serves as a defining element of the human being".
The "pre-ordained duality of man and woman" had to be respected, he said, if families and children were not to lose their place and dignity.
People could not become what he called "abstract human beings" choosing for themselves what their nature would be, added.
In some countries, the Catholic Church has already joined forces with Jews, Muslims and members of other religions to oppose the legalization of gay marriage, in some cases presenting arguments based on legal, social and anthropological analyses rather than religious teachings.
Significantly, the pope specifically praised as "profoundly moving" a study by Gilles Bernheim, France's chief rabbi, which has become the subject of heated debate in that country.
Bernheim, also a philosopher, argues that homosexual rights groups "will use gay marriage as a Trojan Horse" in a wider campaign to "deny sexual identity and erase sexual differences" and "undermine the heterosexual fundamentals of our society".
His study, "Gay Marriage, Parenthood and Adoption: What We Often Forget To Say", argues that plans to legalize gay marriage are being made for "the exclusive profit of a tiny minority" and are often supported because of political correctness.
In his own speech on Friday, the pope repeated some of the concepts in the Bernheim study, including an assertion that children raised by gay couples would be more "objects" than individuals.
Franco Grillini, a leader of Italy's gay community, called the pope's words "great foolishness," saying: "Where gay marriage has been approved, there has been no consequence on heterosexual marriage".
Last month, voters in the U.S. states of Maryland, Maine and Washington state approved same-sex marriage, the first time marriage rights have been extended to same-sex couples by popular vote.
Same-sex unions have been legalized in six states and the District of Columbia by lawmakers or courts.
In November, Spain's highest court also upheld a gay marriage law, and in France the socialist government has unveiled a draft law that would allow gay marriage.