Any one particular church can choose to union whichever particular unions it wishes to grant within the confines of the law. The ability of gays to marry within the confines of the law doesn't mean any particular church has to grant/perform the union itself.
Marriage, to the extent that it exists, would seem to predate any particular "church" doctrine on the matter, since the concept of a contractual union between two people, even if primarily for the purposes of child-bearing or property-swapping, is found in many cultures without "churches". If religious-entities wish to give up in its entirety, the lay power to confirm a legal union, then I'm sure they can probably keep their word "marriage" for themselves (not that they would ever give that up).
Maybe I'm wrong, but the power already isn't the "church's". You still have to have a license from the government.