I actually think this was the intent of the homo "marriage" movement in the first place. The legal definition of marriage will be continually broadened until it becomes meaningless (not that homo "marriage" hasn't already rendered the legal definition meaningless).
Then legal marriage can be done away with altogether, and the traditional notion of the family as well. With the destruction of the family comes the transformation (i.e., destruction) of society. The State will then usurp the role of the family in society, resulting in a socialistic paradise.
I can't wait.