Pope Benedict, starting a trip to Spain where gay marriage became legal last year, said today that there are certain things to which the Church must just say "no".
The 79-year-old pope arrived in Valencia close an international meeting of Catholic families and comfort residents of the city still mourning a train accident that killed 42 people on Monday.
Speaking to reporters on his plane from Rome, the pope was asked about the gay marriage law and other changes in Spanish society that have been challenged by the Roman Catholic Church.
He said he wanted to stress the positive aspects of family life in Spain and elsewhere but added that there were some problems that could not be ignored.
"We have families that love each other, who are happy, we want to stress this reality, which gives hope for the future," he said.
"It is true that there are certain things that Christian life says 'no' to. We want to make people understand that according to human nature it is a man and a woman who are made for each other and made to give humanity a future," he added.
The Spanish law gives gays the same rights as heterosexuals who marry, including on adoption and inheritance. Since the law was passed last year, 4,500 gay couples have married, according to gay, lesbian and transsexual organisation FELGT.
The law put Spanish Prime Minister Jose Luis Rodriguez Zapatero on a collision course with the Catholic Church, which has slammed gay marriage as an expression of "anarchic freedom".
The Vatican has, instead, praised heterosexual families as "a pillar of humanity".
"Let's shine a light on these positive things so we can try to make people understand why the Church cannot accept certain things but at the same time wants to respect people and help them," the pope told reporters.
At the airport, where he was greeted by King Juan Carlos, Queen Sofia and thousands of pilgrims waving flags and singing, the pope said the family, based on marriage, was "a unique institution in God's plan."
"The Church cannot fail to proclaim and promote its fundamental importance," he said. A senior Vatican source on the plane said there was a "certain irritation" within the pope's entourage over Mr Zapatero's decision not to attend a Sunday mass, which will end the rally with more than a million pilgrims expected.
The source noted that in the past, Cuba's Communist leader Fidel Castro, Nicaragua's Daniel Ortega and former Polish President Wojciech Jaruzelski attended masses presided over by the late Pope John Paul when he visited their countries.
Mr Zapatero is due to meet the pope in private at 5.30pm.