178. Some couples are unable to have children. We know that this can be a cause of real suffering for them. At the same time, we know that “marriage was not instituted solely for the procreation of children… Even in cases where, despite the intense desire of the spouses, there are no children, marriage still retains its character of being a whole manner and communion of life, and preserves its value and indissolubility”.(Gaudium et Spes 50)
This is an important distinction. For Christians, and even for non-Christians, marriage is ordered to the spiritual life. Marriage directs a person and her or his life-partner toward God in a way other avenues cannot. Marriage appeals to the basic human impulse for companionship. Marriage engages a believer and a companion to support one another in the pilgrimage to the Divine.
So too, “motherhood is not a solely biological reality, but is expressed in diverse ways”.(Aparecida 457)
Another important point. A person can function as a parent with no biological connection at all.
For your reference, remember that Amoris Laetitia is online here.