The reason the state gets involved in marriage is because strong families are the bedrock for a functional society.
It is certainly possible to rearrange society around other intimate social groupings.
As someone who wasn't a US citizen living in the USA until relatively recently, being married conferred a sense of legal security that my ever changing legal status could be buffeted with. It isn't that I need the state to confer value to my relationship; but rather that the state is messed up and I need a way to live within the patchwork of public policy and laws that otherwise would make my life, and that of my family and wife, all the more difficult.
Perhaps after the revolution marriage as a legal construct will be moot. But until then, I fully support gay marriage as a way to confer legal status to what should always have been there.
"The worker has the right to leave his boss, but can she do it? And if she does quit him, is it in order to lead a free life; where she will have no master but herself? No, she leaves to sell herself to another employer. She's driven by the same hunger. Thus the worker's liberty is only a theoretical freedom, lacking any means of realization; an utter falsehood."