The pope's statement was tied to World Day of Peace, which is celebrated Jan. 1, the Italian news agency ANSA reported.
Benedict said the Roman Catholic position on same-sex marriage is based, not on church teaching but, on the principles of natural law, "inscribed in human nature itself, accessible to reason and thus common to all humanity."
He said that makes the church's opposition more important.
"Efforts of this kind are all the more necessary the more these principles are denied or misunderstood, since this constitutes an offense against the truth of the human person, with serious harm to justice and peace," Benedict said.
The pope's message was titled "Blessed are the peacemakers," a phrase from Jesus Christ's Sermon on the Mount.