Marriage is the free choice of a man and a woman to make themselves irreplaceable to each other. The word has the appropriate finality about it and therefore is obviously life-long. If a man and woman really choose to make themselves irreplaceable to each other, the choice is obviously irrevocable or the spouses were never irreplaceable at all. Irrevocability is another important part of marriage reality that people seldom think of today. Another dimension of marriage reality related to irreplaceability, which recognizes the uniqueness of a particular person, is non-substitutability. No one can be substituted for a spouse as if he or she was your spouse.
The reality of irreplaceability is seen most clearly through the eyes of the child. We each carry the flesh of our mother and father for all eternity. More than just a biological artifact, there is a sense of loss if we are deprived of the love and relationship with our mother and father and the unity between them. These common desires and longings can be called an experience of God’s plan for creation.
In reality, the child is irreplaceable to both the mom and the dad, and the mom and dad are irreplaceable to the child. This makes it obvious: the mom and dad are therefore irreplaceable to each other through their children. Viewed in terms of this reality, it is also obvious that it was the choice to marry that has prepared the man and woman to receive this new person into a circle of irreplaceability.