Should the government end the marriage thing as we know it?
No, keep it as an option for all citizens. As has been pointed out elsewhere, laws have been written that apply only to married people, the two examples that immediately come to mind is a spouse not being forced into testifying against the other spouse and rights to SS payments.
Should we go to contractual agreements only- without the current "default" we have now?
Same as above. How would one contract for SS payments or claim the right to not testify? I don't think it can be done. You don't get to write your own laws without going through the appropriate legislature.
As for people living in polygamous communes, go ahead. Just don't expect to have rights such as child support, alimony payments, and whatever other rights there are to marriage and its subsequent dissolution. In the common vernacular, you're shaking up. It's an informal thing that might involve contracts but usually doesn't.
Neither of those are necessary. If marriage were abolished, then those laws wouldn't necessarily even need to be amended. The condition would simply no longer be met. Ditto for testimony. Things which should be considered in the discussion on whether to abolish marriage, but certainly not conditions that make abolishing it either difficult or impossible. The benefits, like any other benefits associated with being married, would simply cease.