The LA Time just published an article with exactly this view:
Let's fight a larger battle, namely to have government catch up to human behavior. That means recognizing the legitimacy of a wide range of consensual, non-exploitative romantic partnerships, each of which should probably have its own distinct label.
In the U.S., the highest priority should be to give official recognition to cohabitation, which is, in effect, renewable short-term marriage. Married households are now in the minority in the U.S., and cohabitation is increasing, especially among the elderly. Cohabitors are wary of lifetime commitments (in part, perhaps, because such commitments so often prove to be illusory in our culture), but many would probably like the option of getting the same rights and benefits during their cohabitation that married people have.
This would be a step toward stabilizing relationships as they actually occur in 21st century America, and perhaps even toward reducing our disgraceful divorce rate. Trying to force all legitimate partnerships into one defective box -- longterm- oppositesexunion -- denies millions of caring partners the benefits of state recognition and sets up millions of others to fail.
Just to show you that this is not a fringe view, look at the author's credentials:
Robert Epstein is a visiting scholar at UC San Diego and the former editor in chief of Psychology Today.
Take it seriously: there are people for whom same sex marriage does not go far enough, and who see it as a stepping stone toward their ultimate goal of a marriage-free world.