Chicago Cardinal Francis George says his opposition to gay marriage stems from gay and lesbian couples not being able to consummate their unions.

Illinois lawmakers, who legalized civil unions for gay couples 2 years ago, are expected to take up the issue when they reconvene for a lame-duck session in January.

Speaking to the Chicago Tribune, George, the head of the Catholic Conference of Illinois and the Archbishop of Chicago, said such unions violate natural law.

“Marriage comes to us from nature,” he said. “That's based on the complementarity of the two sexes in such a way that the love of a man and a woman joined in a marital union is open to life, and that's how families are created and society goes along. … It's not our doctrine. It's not a matter of faith. It's a matter of reason and understanding the way nature operates.”

“You want to be sure that everybody has a chance at happiness. That's a very persuasive argument,” George continued. “But we all want that, and nobody should be disdained or persecuted because of their sexual orientation. … But when we get behind the church and behind the state, you've got a natural reality that two men or two women … cannot consummate a marriage. It's a physical impossibility.”

George apologized in January for comments he made comparing gay rights activists to the Ku Klux Klan.