I presume the christian institution had something to do with the belief that God is everywhere so he would always be witness to any marriage. Practically however it was still a private affair, and prior to Christianity marriage was a simple contract between two people and had nothing to do with religion.
It's only in recent years that this has become the case, in the past two people making a contract for life made perfect sense because by the time they had some kids together and raised them to adulthood they would be about ready to drop dead anyway. Nowadays we live a hell of a lot longer, and no one get married anymore to "continue their line" so spending all of that long life with one person is somewhat unnatural.
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