I am actually in favor of the proposition that consenting adults, in whatever combination or number, be allowed to enter into whatever contract they desire (except for contract that would be otherwise illegal on other grounds, such as slavery contracts). The best solution would be for the state to simply get out of the marriage game.
The state does have an obligation to uphold morals, like them or not. Society will not exist (as has been proved time and time again) when the state gives up on keeping morals in society.