Chapter 2 of "Sex and Society in the World of the Orthodox Slavs" discusses Medieval Slavic marriage practices and laws.
Review of Sex and Society in the World of the Orthodox Slavs, 900-1700,
Eve Levin,
Cornell University Press, 1989
Arranged child marriages were very common between 900 and 1700 ad. Eve Levin outlines several reasons why these might be to the benefit of families and society:
Remarriage was discouraged, but often adults were permitted up to three marriages during a lifetime. However, second and third marriages were regarded with suspicion. The wedding ceremonies were accompanied, not by celebration, but by prayers for forgiveness of sin. In addition, penance and fasting was required; the number of years' penance to be done increased with the number of previous marriages.
As first marriages were usually arranged, it was through second marriages that couples could pursue more personal interests. While the family was still a major deciding body over the choice of spouse, adults interested in remarrying often did so with some degree of self-interest. Otherwise, they could join the church and practice monastic celibacy.