Catechism of the Catholic Church §1610 — excerpt
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.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.