VATICAN CITY (Reuters) - 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 "Apostolic Exhortation" Benedict said all believers had to defend what he called fundamental values but that the duty was "especially incumbent" on those in positions of power.
He said such values 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 severely divided over the issue of whether to give more rights to unmarried couples, including homosexuals.
Italian politicians from the right and center praised the pope but leftists criticized him.
Franco Grillini, a homosexual parliamentarian and leading gay rights activist, accused the Pope of launching a "moralistic dictatorship based on the fear of sex". Leftist Senator Rina Gagliardi called it "improper interference" in Italy's affairs.
The Pope's words were also applicable to countries like the United States, where some Catholic politicians have said they are opposed to abortion but feel bound to support pro-choice legislation because they represent many people.
In the 2004 U.S. presidential campaign, when Democratic candidate John Kerry, a Catholic, supported abortion rights.
The Pope implied local bishops could not turn a blind eye to such politicians. "Bishops are bound to reaffirm constantly these values as part of their responsibility to the flock entrusted to them," he wrote.
Some bishops in the United States have refused to give communion to Catholic politicians who back abortion rights.
The Pope also reaffirmed the Church's law on celibacy in an all male-priesthood, calling it "a priceless treasure".
Liberal Catholic groups have called for celibacy to become optional for priests in the Catholic Church, saying this would help ease the shortage of priests in many areas.
Benedict re-affirmed that Catholics who divorce and remarry cannot receive communion. The Church does not recognize divorce.
In another section, Benedict lamented that many Catholic priests did not know Latin, the official language of the Church.
He said Latin should be used in parts of large open-air masses held at international gatherings "to express more clearly the unity and universality of the Church".
He said he wanted to see more Latin and more Gregorian chant used in Church services.
"Certainly, as far as the liturgy is concerned, we cannot say that one song is as good as another," he said.