Catechism of the Catholic Church §2378 — excerpt
A child is not something owed to one, but is a gift. the "supreme gift of marriage" is a human person. A child may not be considered a piece of property, an idea to which an alleged "right to a child" would lead.
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: CDF, Donum vitae II, 8