Catechism of the Catholic Church §1628 — excerpt
The consent must be an act of the will of each of the contracting parties, free of coercion or grave external fear. No human power can substitute for this consent.
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: CIC, can. 1103 · CIC, can. 1057 # 1