The Pope warned of the threat posed by same-sex marriage as part of an annual message about peace.
The World Day of Peace as celebrated by the Catholic Church is on New Year's Day, and Pope Benedict XVI released his message for that day on Friday, warning that any efforts to allow gays and lesbians to wed "actually harm and help to destabilize marriage."
The Pope wrote that in order to pursue peace, "There is also a need to acknowledge and promote the natural structure of marriage as the union of a man and a woman in the face of attempts to make it juridically equivalent to radically different types of union; such attempts actually harm and help to destabilize marriage, obscuring its specific nature and its indispensable role in society."
Pope Benedict seemed to claim same-sex marriage violates human nature. After railing against abortion and marriage equality, he said what the other side pursues is "an offence against the truth of the human person."
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"These principles are not truths of faith, nor are they simply a corollary of the right to religious freedom," he wrote. "They are inscribed in human nature itself, accessible to reason and thus common to all humanity."
The Catholic Church in the United States has put money into backing its beliefs, having spent $2 million fighting marriage equality in four states this election cycle.
Meanwhile, American Catholics are moving further away from Pope Benedict on the issue. A new Quinnipiac University poll released this month found that among white Catholics, support for marriage equality is at 49%, with 43% opposed.