The battle around notions of marriage is missing some fundamental definition. We should note that marriage is conducted under two formalisms or magisteria. I use the term "magisterium" in the same sense that Steven J. Gould used it when he wrote about Non-Overlapping Magisteria.
First, the magisterium of religious doctrine. Marriage under Judeo-Christian doctrine, for instance, has a certain view that is commonly interpreted to mean monogamous male-female bonding. In this domain, marriage is a ceremony and a lifelong arrangement conducted and maintained under church oversight.
The second magisterium is civil law. The state issues a license to the parties to be married for several reasons. Recognition of legal marriage under civil law serves to assure that the individuals involved have met certain norms in socially acceptable behavior. Incestuous or plural marriage can be intercepted at this point.
But the state has a compelling interest in other ways. Issuance of a marriage certificate sets an official time stamp to a legally binding relationship. A married couple are a kind of legal partnership with shared privileges, liability and responsibilities. Since the state may be called to settle conflicts relating to divorce, the distribution of assets, and the disposition of dependents, the state resolves the relationship to a legally recognized partnership that is subject to statutory oversight.
The religious magisterium is the realm of the supernatural. This realm is notoriously difficult to prosecute, given the practical difficulty in summoning the deity for testimony. The civil magisterium is much easier to manage and can be done in a largely secular manner.
Where this same-sex marriage movement has gone off the rails is the matter of domain. Those with an orthodox view of marriage will object that the proposed same-sex "marriage" arrangement does not meet the basic definition of marriage, as they understand it. With the religious orthodox folks, not only does the expanded definition of marriage exceed the long understood boundaries, it is wrapped in concepts of sexuality that they can neither accept nor imagine.
What should have happened from the beginning is that the term "marriage" be decoupled from definition of same-sex unions because they are just not taxonomically equivalent.
I am in favor of same sex unions. Period. But I have to say that defining a same-sex union as indistinguishable with male/female "marriage" is just poor taxonomy. Supporters of same-sex marriage appear to believe that marriage is a genus for which there are two species: male/female and same sex. I would argue that the biological arc of the two varieties of coupling really do not fall under the same genus. They are perhaps different genera.
What proponents of same-sex "marriage" should do is deconvolute the argument. Remove the religious dimension for a later time and focus on the secular aspects of marriage as civil union. Words matter, and to try to assert that a large fraction of the population is in possession of the wrong definition of something so deeply wrought with meaning and tradition as "marriage" is counterproductive to the movement.