A new Mexico City law goes into effect March 4 that will allow same-sex couples to marry and adopt children, propelling the city to the forefront of the global gay rights movement.

A backlash is attempting to have the law declared unconstitutional.  One argument being made is that Mexico City already has civil unions for same-sex couples.

That argument, however, just raises the issue of whether government should provide marriage at all; it could instead provide civil unions to both same-sex and opposite sex couples, leaving marriage to religious institutions.