Marriage under the pedagogy of the Law
Moral conscience concerning the unity and indissolubility of marriage developed under the pedagogy of the old law. In the Old Testament the polygamy of patriarchs and kings is not yet explicitly rejected. Nevertheless, the law given to Moses aims at protecting the wife from arbitrary domination by the husband, even though according to the Lord's words it still carries traces of man's "hardness of heart" which was the reason Moses permitted men to divorce their wives.
Source: Catechism of the Catholic Church (Vatican.va)