Pope Benedict on Tuesday linked the Church's opposition to gay marriage to concern about the environment, suggesting that laws undermining "the differences between the sexes" were threats to creation.
The pope made his comments in an address to diplomats in his yearly assessment of world events. The main theme of the address was the environment and the protection of creation.
"To carry our reflection further, we must remember that the problem of the environment is complex; one might compare it to a multifaceted prism," he said.
"Creatures differ from one another and can be protected, or endangered, in different ways, as we know from daily experience. One such attack comes from laws or proposals which, in the name of fighting discrimination, strike at the biological basis of the difference between the sexes," he said.
"I am thinking, for example, of certain countries in Europe or North and South America," he said.
This was a clear reference to legislation either enacted or proposed in several part of the world.
Last month, Mexico City became the first capital in Catholic Latin America to allow same-sex marriage.
In California, the U.S. state's ban on gay marriage goes to trial on Monday in a federal case that plaintiffs hope to take all the way to the U.S. Supreme Court and overturn bans throughout the nation.
Gay marriage is legal is several U.S. states and some European countries.
"Yet freedom cannot be absolute, since man is not God, but the image of God, God's creation. For man, the path to be taken cannot be determined by caprice or willfulness, but must rather correspond to the structure willed by the Creator," he said.
In his speech to diplomats from more than 170 countries, the pope repeated the themes of his message for the Church's World Day of Peace on January 1, which said industrialized nations must recognize their responsibility for the environmental crisis, shed their consumerism and embrace more sober lifestyles.