The Pope took his opposition to gay marriage to new heights today, denouncing what he described as people manipulating their God-given gender to suit their sexual choices.
He said they were destroying the very "essence of the human creature" in the process.
Benedict XVI made the comments in his annual Christmas speech to the Vatican bureaucracy - one of his most important speeches of the year.
He dedicated it this year to promoting family values in the face of vocal campaigns in France, the United States, Britain and elsewhere to legalise same-sex marriage.
In his remarks, Benedict quoted the chief rabbi of France, Gilles Bernheim, in saying the campaign for granting gays the right to marry and adopt children was an "attack" on the traditional family made up of a father, mother and children.
"People dispute the idea that they have a nature, given to them by their bodily identity, that serves as a defining element of the human being," he said. "They deny their nature and decide that it is not something previously given to them, but that they make it for themselves."
"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.
It was the second time in a week that Benedict has taken on the question of gay marriage, which is dividing France after proponents scored big electoral wins in the United States last month.
In his recently released annual peace message, Benedict said gay marriage, like abortion and euthanasia, was a threat to world peace.
After the peace message was released last week, gay activists staged a small protest in St. Peter's Square.
Church teaching holds that homosexual acts are "intrinsically disordered," though it stresses that gays should be treated with compassion and dignity.
As Pope and as head of the Vatican's orthodoxy watchdog before that, Benedict has been a strong enforcer of that teaching: One of the first major documents of his pontificate said men with "deep-seated" homosexual tendencies shouldn't be ordained priests.
For the Vatican, though, the gay marriage issue goes beyond questions of homosexuality, threatening what the church considers to be the bedrock of society: a family based on a man, woman and their children.
But the Vatican's opposition has been falling on deaf ears. Under then-Socialist leader Jose Luis Rodriguez Zapatero, the largely Roman Catholic Spain legalised gay marriage.
Earlier this month, the British government announced it will introduce a Bill next year legalising gay marriage, though it would ban the Church of England from conducting same-sex ceremonies.
In France, President Francois Hollande has said he would enact his "marriage for everyone" plan within a year of taking office last May.
The text will go to parliament next month. But the country has been divided by vocal opposition from religious leaders, prime among them Bernheim, as well as some politicians and parts of rural France.
The Socialist government's plan also envisions legalising same-sex adoptions. Benedict quoted Bernheim as denouncing that in his view, under the plan, a child is now essentially considered an object people have a right to obtain.
"When freedom to be creative becomes the freedom to create oneself, then necessarily the Maker himself is denied and ultimately man too is stripped of his dignity as a creature of God," Benedict said.