Obviously before there was organized religion, people lived together for purposes of having children and carrrying on the human race. In the middle ages, civil and church marriages began to be recorded by the local authorities and the church but it was expensive for the poorer classes to have either a civil or church ceremony and so the ritual of Handfasting was developed by the Celtics in Scotland and Britain - the couple stood together and their hands were placed together with a lightly tied rope -- and to be considered married, they simply had to pledge their love and union in front of witnesses. Eventually, in Scotland, handfasting evolved so that it became a sort of "engagement period" between a couple and it was used by both the poorer and wealthy classes. The couple would pledge their love and commitment in front of witnesses and they would live together for a year and a day. If, during that year and a day, a child was conceived, then the child would be a legitimate heir and the couple could not separate (they were considered married forever). If no child resulted from the union after the year and a day then the couple could choose to separate or stay together. Handfasting was outlawed in England around 1750 but was still recognized in Scotland through the 1930's. Other European countries recognized handfasting in various forms -- for example, in Germany, a wealthy married man was permitted to enter into a handfast relationship with his mistress -- their left hands were bound by rope during the ceremony where they pledged love and devotion and although the mistress would never be recognized on the same level as a wife, her children would be considered legitimate after the children of the lawful wife. There is a lot of information about Handfasting on the internet if you perform a search -- but this is where ancient ideas of living together before marriage began. We now refer to them as pagan rituals but these traditions were practiced widely in the middle ages. http://www.legalpaganweddings.com/page5.htm
I hope that information gives you a starting point. Please let me know if you have any further questions.