That would be a good solution.
But even though marriage is often done as a religious rite, isn't it also a 'contract" that has a lot of financial implications?
All those issues about inheritance, powers of attorney, etc, etc. Can all those be resolved without gov't involvement?
I remember when AIDS was first around, gay partners often were denied access to their dying loved ones because of family interference. It was an ugly time.
How does the partner get the privileges of marriage in your scenario? If that's resolved, then I'm on board.
The marriage vows talk about "forsaking all others". I suppose that means other partners; but it also might imply putting spouse above family too.
Get that figured out without the gov't, and the churches, and we're good to go.