Chicago's Cardinal Francis George and his six auxiliary bishops officially entered Illinois' gay marriage fray Tuesday, issuing a letter that urges parishioners to contact state legislators and voice opposition to a legalization bill that could face a vote this week.

"Civil laws that establish 'same-sex marriage' create a legal fiction," George and the bishops wrote in a letter sent to priests Tuesday. "The state has no power to create something that nature itself tells us is impossible."

... While traditional marriage advocates have cited Scripture as the basis of their objections to civil unions and gay marriage, Roman Catholic leaders have been highlighting their belief that same-sex relationships violate natural law.

According to the tradition of natural law, every human being must seek a fundamental "good" that corresponds to the natural order to flourish. Natural-law proponents say heterosexual intercourse between a married man and a woman serves two intertwined good purposes: to procreate and to express a deep, abiding love. For that reason, they say, homosexual relationships are not equal to heterosexual ones.