If marriage is for the benefit of society, then the author is correct. If marriage is for the benefit of the couple (or whatever), then it's not needed at all. Either way, those that want to redefine marriage as whatever anyone wants it to be are simply wrong. If they get their way, the result won't be a broader opportunity for alternative lifestylists to marry; it will be the eventual abandonment of marriage.
Of course, (1) the communist movement has been espousing the end of marriage as a societal institution since at least the mid-19th century, and (2) the US is already on the road to achieving this utopian-communist ideal (the marriage rate in the US has dropped from over 80% in just 40 years).