Matthew 22:21Render therefore unto Caesar the things which are Caesar's; and unto God the things that are God's.
The holy marriage ceremony belongs to the faithful and to the church, so the government should stay out of it completely. That prevents the institution of marriage from being profaned by forced acceptance of that which is abbhorent to the faithful.
The rest of the equation is the realm of the law and the courts. This includes child custody, inheritance, common property, spousal benifits, etc.. This should be the subject of a private contract of domestic partnership between any adults who wish to enter into such a contract, be they hetro, homo, polygamist, or whatever. The only role the government has is in writing probate, child protection and joint tax filing laws and regs, which a domestic partnership contract must take into account if it is to be valid.
Under this system, God keeps his, Caesar keeps his, and everyone is free to do as they wish.