Benedict repeatedly referred to marriage as a union between man and woman in an address to a conference of the Diocese of Rome on the role of the family held at St. John Lateran basilica.
He said matrimony was not just a "casual sociological construction" that changed in certain times in history but rather an institution that had its roots "in the most profound essence of the human being."
"The various forms of the dissolution of matrimony today, like free unions, trial marriages and going up to pseudo-matrimonies by people of the same sex, are rather expressions of an anarchic freedom that wrongly passes for true freedom of man," he said.
The Vatican defines matrimony as a divine union between man and woman.
Benedict, the former Cardinal Joseph Ratzinger, spearheaded a Vatican campaign against same-sex unions in 2003, issuing guidelines for Catholic politicians to oppose laws granting legal rights to gay couples when he was prefect of the Congregation for the Doctrine of the Faith.
The former cardinal also frequently voiced the church's opposition to abortion — a message he repeated Monday.
Children, he said, were the fruit of marriage and reflected God's love for man.
"From here it becomes all the more clear how contrary it is to human love, to the profound vocation of man and woman, to systematically close their union to the gift of life, and even worse to suppress or tamper with the life that is born," he said.