Supreme Court Same Sex Marriage Argumet

[Dystopia]

This is where my closet libertarian creeps in and suggests that the people involved in the marriage create the contract themselves, and only go to the gov't when the contract is breached, instead of the gov't setting the rules for who get's to be 'married'.

I couldn't disagree more - People can't legally create new contracts, that's not how it works. I'll explain briefly why:

We all want (I think) to live peacefully and that demands the creation of laws that dictate how we should and shouldn't behave - What we can do, what we should do, what we totally can't do, grey areas, etc. For a Law to be a real Law it needs some form of authority, otherwise no one needs to obey it - What is the institution that holds enough authority to enforce a sect of Laws? That's right, the State/government (I personally prefer calling it the State because there's more to the State than just the government). 

Obviously, this means every contract that exists and is legally valid must be allowed by the State in some sort of Law, written or not - Commonlaw and Civil/Legal systems differ but the belief that there must be some authority to enforce and create laws remains - The State (and the courts of Law, which are a part of public sphere) seems to be the most appropriate entity to create rules/laws. 

This is why people don't decide what a contract is - If people decide what marriage is, then everyone can decide anything that pleases them, and there may be no certain definition, therefore marriage and other contracts become undefined and cease to exist in practise.

Also, if the government doesn't have the power to dictate some basic rules of the contract, why should it have a say when it is breached? It doesn't make sense.

I'm not arguing against polygamy, I'm just saying your notion about marriage is wrong - If we want to legalize polygamy, the State and parliaments/politicians need to pass a Law saying it is legal. As for my stance, I agree with Prryho that it would create many legal barriers that are either very hard or impossible to withstand - In Europe, where the State has a larger, older tradition of intervening more with life's affairs, including regulating institutions like marriage (and most Laws are written and approved by the parliament, a tool of the State) it is an even harder achievement because there are thousands of Laws meticulously regulating how marriage and other institutions work, what you can and can't do, how divorce, child custody, inheritance and portions of it work, what legal benefits each matrimonial regime (like separation of property), etc.

For these reasons, I think if we want to legalise polygamy, I think we are better off following the communist advice of abolishing marriage and we could create a whole new institution from scratch that allowed more freedom and liberty - I think this is crucial because considering the history and traditions associated with the institution of marriage in most of the west and part of the rest of the world it is almost impossible to make a good case for polygamy without entirely (I mean this literally) changing the institution.

There's more to it that simply saying "consenting adults can do what they want" - No they can't. I can't consent on a lot of things, even as an adult, and there are rules and purposes for any contract.