Archbishop Vincenzo Paglia, head of the Pontifical Council for the Family, also said the church should do more to protect gays and lesbians from discrimination in countries where homosexuality is illegal.
In his first Vatican press conference since his appointment as the Catholic church's "minister" for family, Paglia conceded that there are several kinds of "cohabitation forms that do not constitute a family," and that their number is growing.
Paglia suggested that nations could find "private law solutions" to help individuals who live in non-matrimonial relations, "to prevent injustice and make their life easier."
Nevertheless, Paglia was adamant in reaffirming society's duty to preserve the unique value of marriage.
"The church must defend the truth, and the truth is that a marriage is only between a man and a woman," he said. Other kinds of "affections" cannot be the foundation for a "public structure" such as marriage.
"We cannot surrender to a sick egalitarianism that abolishes every difference," he warned, and run the risk of society becoming a new "Babel."
France is in the process of legalizing same-sex marriage despite fierce opposition from the Catholic church; a similar fight is brewing in Britain, with the Anglican and Roman Catholic churches sharply opposed to the move.
In a September document on gay marriage, French bishops recognized the value of France's current civil unions law, which grants heterosexual and homosexual couples some benefits, such as tax breaks.
In November, voters approved gay marriage by popular vote in Maine, Maryland and Washington state, and the U.S. Supreme Court will hear arguments this spring over federal and state bans on gay marriage.
Responding to journalists' questions, Paglia also strongly condemned discrimination against gay people, who he said "have the same dignity as all of God's children."
"In the world there are 20 or 25 countries where homosexuality is a crime," he said. "I would like the church to fight against all this."