I actually think this is conceptually simple. Making all the changes to actually accomplish it....
Essentially, the government marries no one. Everyone gets civil unions.And the legal consequences of civil unions are identical to the current legal consequences of marriage. This includes all those interstate reciprocity features that civil unions don't currently have. Really the only thing that same-sex couples don't get is the word "marriage", or at least they don't get it from the government. The thing is, no one gets it any more.
If you want to be "married", you do what our forebears did generations earlier - you go to your church. And it's entirely up to your church whether they do or don't marry you. And if you do go get "married", don't forget that to the government that means nothing at all. If you want those legal entanglements, you must separately go through the hoops for a "civil union".
This is really nothing more than re-separating church and state in an area where they became entangled.
