Germany's bishops' conference has expressed regret over a parliamentary vote to allow same-sex marriage, vowing to defend the "Catholic understanding of marriage."

The vote by lawmakers "abandons the differentiated perception of various forms of partnership in order to stress the value of same-sex partnerships," said Archbishop Heiner Koch of Berlin, chairman of the bishops' Commission for Marriage and Family.

"But differentiation isn't discrimination, and same-sex cohabitation can be valued through other institutional arrangements without opening up the legal institute of marriage," he said.