So why the concept of “marriage” at all at the governmental level?

You have very good points, but the one thing that I can't think of a way to address without a concept of marriage is the homemaker. As an employee of a company that provides benefits to me and my family, I can't think of a way to get the government to define the fact that they also are required to provide benefits to my wife without calling her my spouse or some arbitrary circumlocution that would essentially mean spouse.