Catechism of the Catholic Church §1647 — excerpt
The deepest reason is found in the fidelity of God to his covenant, in that of Christ to his Church. Through the sacrament of Matrimony the spouses are enabled to represent this fidelity and witness to it.
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.