Vatican City - The Church's opposition to gay marriage is "non-negotiable" and Catholic politicians have a moral duty to oppose it, as well as laws on abortion and euthanasia, Pope Benedict said in a document issued on Tuesday.
In a 140-page booklet on the workings of a synod that took place at the Vatican in 2005 on the theme of the Eucharist, the 79-year-old German Pope also re-affirmed the Catholic rule of celibacy for priests.
In the document, known as an "Apostolic Exhortation," Benedict says all believers had to defend what he calls fundamental values but that the duty was "especially incumbent" for those in positions of power.
He said these included "respect for human life, its defense from conception to natural death, the family built on marriage between a man and a woman, the freedom to educate one's children and the promotion of the common good in all its forms."
"These values are not negotiable," he said.
"Consequently, Catholic politicians and legislators, conscious of their grave responsibility before society, must feel particularly bound, on the basis of a properly formed conscience, to introduce laws inspired by values grounded in human nature," he said.
Gay marriage is legal in several European countries, including predominantly Catholic Spain, and Italy is currently severely divided over the issue of whether to give more rights to unmarried couples, including homosexuals.
The Pope's words in the document were also applicable to countries such as the United States, where some Catholic politicians have said they are personally opposed to abortion but felt bound to support pro-choice legislation because they represent many people.
Some bishops in the United States have refused to give communion to Catholic politicians who back abortion rights.