When you stand before the altar and you promise to God that you are freely giving yourselves to one another and will be both faithful and fruitful, there's a certain expectation that your vows will lead to parenthood. When years go by and empty arms are all you have to show for it while friends and family all around you have babies to take care of and children to raise, you can begin to feel a woundedness of heart. You may wonder if God is somehow judging you as unfit to be a parent which is doubly hard to reconcile when you see how many people aren't taking care of the kids that they seem to have in such abundance.
You may cry and rage against God's unfairness. It may seem like somehow your prayers have been forgotten and you aren't really loved by Him. There may be a time when the sight of a pregnant belly, a crying infant, or children on a playground is too painful for you to be near. You listen to friends who are parents complain about the precious children they have and you bite back the bitter words that threaten to spill forth from your lips about how you would gladly take them off their hands. All the while you wonder why. All the while you question, you doubt, and you struggle to make sense of the whole thing.
Eventually, you come to terms with the fact that you aren't going to have children, and you ask God how you are supposed to be fruitful when every effort you make to be fruitful is unproductive. That's when you begin to realize that you weren't blessed with children from your own body, but God has many children who need you. He has chosen you for a mission of even greater impact: to be a surrogate parent to the children of those who surround you. You start to open your heart to the orphaned, to the neglected, to the abandoned children around you. You realize that you can't literally take them all in but you can pray for them, be a friend, offer them the shelter and support they need from a world that often offers them empty promises and false hopes rather than real, authentic truth.
To the infertile couple, your true vocation is to be parents to all children. You aren't limited to a specific set of them, but to become the parents of the lost and the downtrodden and the wayward who have no one else to care. You realize that your lack of your own children gives you a unique flexibility to pick up at a moment's notice and be there in ways that someone with their own children cannot. You come to see that infertility, rather than being a curse, is a gift that opens the heart wide open to the children all around you who are confused, neglected, and lonely. And you begin to praise God for what He has done in your life because none of it could have happened without your infertility.