We know that every human right protects a common good. What is a common good?
A mother and father are "common" and "good." They are "common," meaning every and therefore each person without exception has a mother and father. They are also "good," which is something that contributes to human flourishing as God created us.
The Church specifically says “The child has the right to be ... brought up within marriage: it is through the secure and recognized relationship to his own parents that the child can discover his own identity and achieve his own proper human development.”1
We all have a desire to know where we came from, who is my mother and father, who are my brothers and sisters, who do I look like, and we want to be connected with them and loved by them. This is an experience of God’s plan for creation.
Procreation is a common good — necessary for the continuation of the family and the human race. Having “children through the responsible exercise of one's sexuality” is another human right, as the Church teaches.2 The word “Responsible” refers to the right of the child to be born into marriage. Therefore, as William E. May explains in his book Marriage the Rock on Which the Family is Built, no one has the right to procreate unless they have first made themselves irreplaceable to each other in marriage.
With every human right comes a corresponding duty.3 Since the child is powerless over his right to be born into marriage, the duty falls to us to promote marriage as the privileged institution that unites children with their mothers and fathers, and we must evaluate laws, institutions and curricula by how well they encourage men and woman to marry before having children. Responding to this duty is the work of justice.
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1 Donum Vitae (Instruction on Respect for Human Life in Its Origin and on the Dignity of Procreation) IIA1. Congregation for the Doctrine of the Faith, 1987
2 Centesimus Annus (On the Hundredth Anniversary of Rerum Novarum) 47 Pope St. John Paul II, Encyclical Letter
3 Pacem in Terris (On Establishing Universal Peace in Truth, Justice, Charity, and Liberty), 28-36. Pope St John XIII