Every healthy child is born with a functioning circulatory system, digestive system and respiratory system. But no one, no matter how healthy, is born with a complete, functioning reproductive system. Every healthy person is born with one half of a reproductive system, either the male half or the female half. And that system is not functioning until the person reaches sexual maturity and unites with a member of the opposite sex.
For most animals, It is a biological marker of adulthood to be able to bring forth offspring. Of course, to properly bring a human child into the world requires a maturity beyond the mere power to impregnate or to conceive and give birth. Human children need to be educated, not only in practical matters but in the virtues, especially in the crowning virtue of love. It is above all in the family, formed through the irrevocable union of husband and wife, that children learn to receive love and to take their first, exploratory steps toward giving themselves to others in love.
It would not be far wrong to say that the purpose of marriage and family life is to help both spouses and children to grow up, to become adults, to become capable of the self-sacrificing love necessary for marriage and family life. The same virtues necessary for marriage and family life are also necessary for those whose vocation does not include marriage. We are all made for love, as Saint John Paul II has said: “Man cannot live without love. He remains a being that is incomprehensible for himself, his life is senseless, if love is not revealed to him, if he does not encounter love, if he does not experience it and make it his own, if he does not participate intimately in it (The Redeemer of Man, 10).”
The spiritual and the material work hand-in-hand in God’s plan for marriage. The spiritual union of husband and wife, and the physical union proper to marriage, are both naturally ordered to a mature love which gives of self for the good of another. Spouses show this love especially by their willingness to embark on the adventure of making each other into parents, with all of the responsibility and “growing up” that exalted office requires. In all of this, spouses help each other (and their children) to become the people God intends them to be. In the process of becoming dad and mom, husband and wife have the chance to become mature adults, with a complete physical reproductive system complemented by a spiritual willingness to educate their children in love.