Guys, I think you are talking past each other. Pair bonding vs formal authority ceremony. The first is a natural / animal act, the second is a money maker and means of exercising political control. If you apply the same phrase "marriage" to both concepts, then it is quite possible to be married without being married...
How long have little girls (for the sake of cultural stereotypes) been longing for the day they say "I do" with Prince Charming or Billy Badass? I don't know the answer.
I don't know when the institution of marriage was implemented. I doubt it was practiced among our cave dwelling ancestors. I don't know if it is a by-product of the agricultural revolution - though that seems to me a possibility. I don't know if it is a by-product of formal / institutional religion, which is not out of the question.
But as long as women have estrus, and men have erections, the two are going to pair bond. That is an organic reaction.
How that came to become an adjunct of religion, I don't have an answer. Psychology is all over the place. Just because someone in Greece or Rome two thousand years ago was in a marriage doesn't mean they (men and women) didn't get a little side action from time to time. I'm not passing judgment here, just stating fact. Formal civil marriage was probably more about power, social status and inheritance, especially among the aristocracy...and why bastard children were sometimes ostracized. I don't think the peasant and slave classes were particularly consumed with these concerns.
I do know in the Roman history I looked into leading up to Constantine and the Battle at Milvian Bridge, one of the Roman Emperors (there were 4 at the time) "sampled" the bride prior to the marriage for a number of his aristocrats. I think it was Galerius but I'm going by memory here.
Point being applying modern sensibilities to historic situations is always going to lead to faulty and erroneous conclusions. It isn't a simple, cut and dried situation, human behavior has always been complex and seldom logical.
Now I need to wipe my feet from the mess I've stepped into...