Oh come on, you really think they're going to add this?
Marriage is the bond between a man and a woman - or at least two people. Not 3. Not 10. Not 478. Graal should be no different. That will just cause problems and parents to complain.
The only thing you could possibly accomplish in this thread is getting marriages removed completely. And I'd hate to see that happen.
This isn't going to happen and to argue it is pointless and a step in the wrong direction.