Roman Catechism 22.21

Chapter 22 — Part II: The Sacraments — Matrimony

Section 21. Marriage As A Natural Contract — As grace perfects nature, and as that was not first which is spiritual, but that which is natural; afterwards that which is spiritual, the order of our matter requires that we first treat of Matrimony as a natural contract, imposing natural duties, and next consider what pertains to it as a Sacrament.