Marriage is the free choice of a man and woman to make themselves irreplaceable to each other, to become a relative, described in scripture as becoming one flesh. It is this choice that prepares the couple, if so blessed, to receive a child as a gift. In reality, that child is irreplaceable to both the mother and the father, and the mother and father are irreplaceable to the child. Though it may not be evident to adults, the child knows that his or her mom and dad are also irreplaceable to each other, even if they living separately.
The mother and father are part of the identity of the child who carries their flesh for all of eternity as Christians know to be true. Our desire to know and be loved by our own mother and father is an experience of God’s plan for creation that is recognized through the reality of marriage. Knowing this universal reality does not depend on belief in God. This reality just is.
True marriage is constituted by Reality (the will of God, the creator of reality) and the will of man who must choose to recognize and enter into it. When the reality of marriage is recognized by culture, religion or the state, it becomes the only institution that unites children with their mother and father.
Through the benefit of revelation, Catholics have a deeper understanding of this reality, and through Christ, marriage has become a sacrament, an outward sign of the hidden reality that is so apparent to the child.
 The choice of marriage also prepares the man and woman to receive a child through adoption. Only a man can stand in for the father and only a woman can stand in for the mother that the child has lost.
 Cf. Casti Connubii #9. Pope Pius XI. 1930