Reading through this whole thing, I've concluded that perhaps the biggest problem is that at least three different definitions of marriage are being used. Two are primary, and the third can be viewed as a consequence of one of those. These are:
1. Marriage is a religious institution, which is why it says "holy matrimony", and thus is the province of the churches.
2. Marriage is a legal document approved by the state, and is therefore the realm of the state; it is thus also an economic institution, since the contract has numerous eceonomic aspects automatically recognized.
Both definitions have considerable merit. It helps to look at the situation historically, because both definitions are also true -- and it is the overlapping and confusing of the two that causes all the trouble.
Marriage predates the state. Before there were even kings, or cities, there was marriage. Almost universally, it was bound up with the gods, or spirits, or God. This tradition -- and definition -- has come down to us today, and is the one the religio-publicans cling to.
Yet the state took up the defining of marriage for reasons of inheritance. Even so, the earliest states which did so were generally theocracies, or close to it, and marriage was whatever the priests/priestesses/shamans/etc. said it was. THus whoever the religious authorities declared married were married, because even though the state had a vested interest, marriage was still in the realm of the religious and under that authority.
Things continued in that fashion through the Roman Empire and into the Medieval period. They remained that way after the Reformation, right on into the colonial period. In the colonies of the Catholic powers, the institution was unchanged from the time of the fall of Rome, except that it had become more important politically and economically and so tied in with the state -- but the religious authorities still ruled it. For the "Protestant" powers the situation was the same -- but there the breakdown of the marriage of the religious and political began. People in the Protestant countries moved about, and took their marriages with them. For the sake of order, the different states tacitly recognized marriages bound in others; still, each state had its own church, and in that state it was that church which solemnized marriages. The single exception was the Jewish community, which had always been separate; their marriages were mostly recognized -- but not always.
England, and then America, was where matters began to change. England had the Anglican Church, but more and more there were Dissenters, who performed their own marriages -- which were recognized by the Crown, thus establishing the principle that marriage could be performed by more than one religious authority. In practice, this transferred the "ownership" of marriage from church to state. The move from definition 1 to definition 2 had been made in all essentials but one: it reamined for marriage apart from any religious authority at all to enter the picture.
As far as I'm aware, that happened in America, for a variety of reasons: One was that there were times and places where competing churches abounded, and yet a couple wishing to be married chose none of them, and so provision was made for a civil marriage. Another was that there were times and places where there wasn't a church available, so the civil authority stepped in. Lastly, there were those who didn't feel the need for any religious aspect at all in their marriage; for them the state sufficed not as a substitute, but as an entity apart.
This all came about because the United States determined to protect the freedom of religion, and thought the thirteen original colonies had among them official state churches, the movement of people from one to another brought about the same practical considerations as in Protestant Europe earlier -- and went beyond it. Official churches were abandoned entirely, the various sects and denominations spread from their home colonies to others, and new ones sprang up. With all this multiplicity, marriage by default became the province of the state, as the only repository handling the records of all marriages. And gradually this system spread back to Europe as religious freedom caught on, notably in Germany, where the official church varied from each tiny principality to the next, and even changed with a change of ruler!
So we come down to today; most of the world follows this pattern. Unfortunately, affairs are muddled; although marriage has for the most part become an affair of the state, the churches still consider it a religious matter, as do many people -- not all of them religious themselves.
So if there is confusion in the definitions in this thread, they arise out of a very real confusion in history and thought.
THe current crisis arises out of it as well: the religio-publicans cling to that ancient situation, and wish to return to it today. They may not say so, but that is what their actions proclaim. Oh, they are gracious enough to share the institution with anyone who agrees with their model -- marriage before God, joining one man and one woman -- but only because it serves their agenda, which at root is an infusion of theocracy into the United States of America.
Ironically, these are the same people who assert that God Himself gave America the form of government it has in the Constitution; for that very Constitution, if they but read what it says, confounds their aim. For if marriage is in fact "holy matrimony", what then are they to do with, "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof"? Certainly their definition of marriage fits in more than one religion, but it does not fit in all. Certainly also that definition has been enshrined in law, piecemeal, over the years (especially since the invention of the Infernal Revenue Service), but that does not make it right. Precedent may give weight, but only when it follows the Constitution; that was decided beginning after the Civil War and reaching its height in the civil rights movement.
So...
I say that MCsNo1Fn has the matter right: forget trying to get in on the marriage racket; go for civil unions -- but don't settle for that! Instead aim for replacing every last mention of marriage in Federal and State law with the words "civil union" -- and let the churches have their "holy matrimony". Certainly any marriage performed by a religious authority would continue to qualify as a civil union, but so would any other ceremony by whoever people agreed qualified -- the Elks Club, the Masonic Lodge... or Sam's Gay Flame Bar and Grill.
And that brings me to the original question. If we truly want freedom, then our battle here is not to steal marriage from the churches -- that's what they see! The battle is instead to allow people of every religion found in America to have marriage, or partnering, or bonding, or whatever they wish to call it, in whatever fashion they consider fit and proper. And if that includes polygamy, or polyandry, or group, or lines, so be it.
After all, it is supposed to be a free country!