The Pope has issued a direct rebuke to David Cameron and other leaders for their attempts to introduce same-sex marriage.
He warned that the move "threatens the family to its foundations".
Benedict XVI dedicated one of his most important public addresses of the year to the theme of the family which he said was under "attack" and "in crisis". In his Christmas address to the Vatican he said moves were afoot which tried to question "the very notion of what being human really means".
In his most outspoken comments on the subject yet, he denounced what he described as people manipulating their God-given identities to suit their own sexual "choices".
He also criticised parents who too often view children as "an object to which people have a right" rather than a gift. It is the second time in a week in which the Pope has spoken against homosexual marriage which is now a central plank of political programmes in Britain, France and a string of American states.
Homosexual activists have been staging protests in Italy after the Pope recently suggested that same-sex marriage was a threat to world peace.
In an official translation of yesterday's (Friday's) address, which was delivered in Italian, he said: "Despite all impressions to the contrary, the family is still strong and vibrant today. But there is no denying the crisis that threatens it to its foundations - especially in the western world."
He quoted a recent essay by the Chief Rabbi of France, Gilles Bernheim, which he said detailed the attack we are experiencing on the "structure of the family, made up of father, mother, and child".
The Pope said: "While up to now we regarded a false understanding of the nature of human freedom as one cause of the crisis of the family, it is now becoming clear that the very notion of being - of what being human really means - is being called into question." He rejected modern arguments about gender.
"According to this philosophy, sex is no longer a given element of nature, that man has to accept and personally make sense of: it is a social role that we choose for ourselves, while in the past it was chosen for us by society.
"They deny their nature and decide that it is not something previously given to them, but that they make it for themselves."
Attempts to question traditional gender roles were as unnatural as the destruction of the environment, he said.
"The manipulation of nature, which we deplore today where our environment is concerned, now becomes man's fundamental choice where he himself is concerned," he said.
The pontiff claimed an obsession with rights had also begun to undermine the special place children held in society.
He said: "The child has become an object to which people have a right and which they have a right to obtain."