In reality, every child is irreplaceable to both their mom and the dad, and their 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 the choice to marry is what has prepared the man and woman to receive this new person into a circle of irreplaceability.
The married couple is not only related to each other through their children, but through their choice to make themselves related to each other through their marriage. “Therefore a man leaves his father and his mother and cleaves to his wife, and they become one flesh.” (Gen 2:24, RSVCE). Turning to the New American Bible for this same verse, the clarifying footnote reads, “The covenant of marriage establishes kinship bonds of the first rank between the partners.”
The validity and dignity of the marriage does not depend on its fruitfulness, but by recognizing the reality of becoming one flesh by free choice to live a vocation of life and love.
Marriage (and adoption) provides the only opportunity to pick our relatives, to make someone irreplaceable. So as the result of the choice for irreplaceability through marriage, the question is no longer a case of picking friends for pleasure, or changing relationships according to changing needs for fulfillment. It is now an irreplaceable relationship between relatives.
Read more: Understanding the marriage "commitment"