I don't know if they could do as you've described. This is about civil marriage. Believe me, though, it would cause a very strong push, in the traditional liberal protestant churches, i.e. the episcopals, the presbyterians, the lutherans, and the methodists to allow homo marriage, thus further ensuring their irrelevance and probable disintegration.
I guarantee you--the Missouri Synod branch of the Lutheran Church will NEVER allow gay marriage in its churches.