I think that where you stand depends on where you sit.
Like you, I suspect that marriage predates written history. If that's the case, then how does one decide whether the framework it took place under was "legal" or "religious" or "social" or ...? I don't think the conclusion is that religion wins.
The Code of Hammurabi (reigned 1795-1750 BCE) http://www.commonlaw.com/Hammurabi.html
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128. If a man has taken a wife and has not executed a marriage contract, that woman is not a wife.
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167. If a man has taken a wife, and she has borne him children and that woman has gone to her fate, and he has taken a second wife, and she also has borne children; after the father has gone to his fate, the sons shall not share according to mothers, but each family shall take the marriage-portion of its mother, and all shall share the goods of the father's estate equally.
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Of course, it's known for being a framework of laws, so it's not really surprising that religion isn't mentioned in the discussion of marriage.
http://en.wikipedia...._marriage_rituals
Chinese marriage became a custom between 402-221 B.C. Despite China's long history and many different geographical areas, there are basically six rituals, generally known as the three letters and six etiquette [...]
In my reading, marriages (and divorce) were a clan-to-clan arrangement. The state got involved in divorce if a crime had been committed by one of the people. Otherwise, there didn't seem to be a state or religious sanction. Instead it seemed to be based on building bonds between families.
In Western societies, non-religious figures have conducted marriages for hundreds of years: ships captains (in some cases, see below), justices-of-the-peace, etc. And in the US, one need not even have someone else administer an oath or conduct a ceremony - http://www.straightd...rry-people-at-sea
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Let's start with the one rock of certainty in this discussion: No state has enacted a statute explicitly authorizing ships' captains to solemnize marriages. However, in ruling on the validity of such marriages, the courts have waffled. On the one hand there is a longstanding legal presumption that if two people think they got married, they did get married, even if the proceeding by which this was accomplished was suspect. On the other hand, judges have also felt, jeez, we can't let just anybody solemnize marriages, we gotta have rules.
This ambivalence has resulted in decisions on both sides of the fence. In Fisher vs. Fisher the court ruled a marriage by a ship's captain valid; in an 1898 case in California, Norman vs. Norman, the court ruled the opposite. It's important to note that in Fisher the court did not specifically single out ships' captains (as opposed to say, mailmen) as having the power to perform marriages; rather it ruled that, absent a statute to the contrary, and subject to certain other conditions, an exchange of vows between consenting parties constituted a valid marriage--as I read it, whether there was an officiant or not. In other words, marriage by ship's captain, or by anybody other than a recognized minister, JP, etc., was a type of common-law marriage.
There are still some states that recognize common-law marriage. Typically all that's necessary is that the parties (1) be legally free to marry (e.g., no undissolved prior marriages); (2) properly consent; (3) "cohabit" (do it); (4) live together; and (5) let the neighbors think they're married. (Contrary to common belief, it is not necessary that the couple live together for seven years.)
What's not required are the services of a minister. So while you're correct in saying "there is authority that a marriage performed by a ship's captain on the high seas is valid," captains don't have any special powers in this regard. A close reading of Fisher suggests the ceremony might as well have been performed by a waiter.
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And so on.
Whether one thinks that marriage is more of a religious or societal/legal institution, I think the "proof" of the proposition will depend on how one conducts the search. The historical record isn't complete, so the answer you get will depend on the assumptions you use.
So, we're here, even if we don't all agree on how we got here.
What seems to me to matter most in this context is: 1) Marriage has existed in our societies for hundreds of years. 2) Much of our legal framework and relationships with government and with the economy are influenced by whether one is married or not. 3) Equal protection arguments are compelling.
My $0.02.
Cheers,
Scott.