State sanctioned marriage has hundreds of years of legal precedence and history behind it, somewhat protected by the tradition of stare decisis.
Let the government scrap that history and tradition and start over with all new law and regulations is just inviting them to rewrite it the way the radicals deem politically correct.
I don't think scrapping state sanctioned marriage would require a total rewrite of the law. We already have existing contract law to cover the binding relationships people might enter.
I guess there would need to be some changes to the tax code for joint filing, to how Social Security benefits are assigned, and to the spousal immunity privileges in trials. None of that seems too significant. Is there any other area of the law that would have to be rewritten in a significant way?