Catechism of the Catholic Church §1646 — excerpt
By its very nature conjugal love requires the inviolable fidelity of the spouses. This is the consequence of the gift of themselves which they make to each other.
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.
Also cites: GS 48 # 1