I have a Kindle book by James Patrick Holding that talked about that solution to the problem; however even he recognizes that it is still a problem, and for two main reasons: one, it doesn't solve the philosophical issue that is at stake on the definition of marriage. It merely pushes the problem to the "civil" sphere, moreover it gives "Caesar" the say on what can be "marriage" without a religious context, which marriage has always been about. That's the point; marriage is primarily a familial union ritual that always had a religious context to it, whether Christian or from the Greco-Roman polytheistic culture. Marriage as a ritual was never devoid of a religious context. To have the State secularize it is itself a problem, especially for a union that is not even legitimate, since marriage has always been understood and defined between a man and a woman.
The other problem is that even some radical LGBT people recognize the inherent inequality between the State "sanctioning" a civil union that the Church is free to delegitimize in its own circles and rhetoric, that's why some gays are so up front in pressuring Churches to recognize gay marriage as marriage itself. Even so, civil union, as philosophically fallacious as I see it, is the best we got to circumvent the problem the SCOTUS introduced at 2015.
