Pope Benedict XVI condemned gay marriage on Friday in a speech to bishops from the United States after the US state of Maryland last week became the eighth in the nation to legalise same-sex unions.

"Sexual differences cannot be dismissed as irrelevant to the definition of marriage," the pope said, warning against "the powerful political and cultural currents seeking to alter the legal definition of marriage."

"Marriage and the family are institutions that must be promoted and defended from every possible misrepresentation of their true nature," he said.

He also said bishops could not overlook "the serious pastoral problem presented by the widespread practice of cohabitation."

"The contemporary crisis of marriage and the family... has led to grave societal problems bearing an immense human and economic cost," he added.

Benedict has frequently warned against liberal family values in veiled references to homosexual marriage and adoptions by gay couples and the Catholic Church often condemns countries that recognise gay rights.

Benedict praised the visiting bishops for their pastoral work.

"I appreciate all that your parishes, schools and charitable agencies do daily to support families and to reach out to those in difficult marital situations, especially the divorced, single parents, teenage mothers and women considering abortion," he said.