Well, in the first place, it’s gone too far for that. Gay marriage advocates would never be satisfied with that option.
More importantly though, the question is based on the false idea that marriage was invented by churches. It wasn’t. Marriage began as a natural institution (men and women deciding on their own to make a lifelong committment to each other) that became a social institution when Hammurabi (late 1700’s BC) distinguished it as separate and distinct from cohabiting, or same sex relationships, or hookups with temple prostitutes etc.
So marriage existed as a natural institution that was raised to a social institution because of the benefits marriage gave to society.
At a much later date, churches give marriage a new spiritual dignity as well, but churches were late to the marriage banquet as it were. The Church, in particular, didn’t invent marriage any more than it invented bread or water. It just uses those naturally occuring things and gives them a new spiritual dimension. To suggest that we “give marriage back to the churches” makes as much sense as giving churches all the water rights in the world, or signing all the bakeries over to the Church.
Marriage evolved as a natural and social institution because it was good for people and good for society. It must be protected for that same reason. That said, if it isn’t protected, marriage won’t die. Society as we know it will. And then a new society will be created by the people who value traditional marriage because it is marriage that drives civilization, not the other way around.