11 Thus amongst the blessings of marriage, the child holds the first place. And indeed the Creator of the human race Himself, Who in His goodness wishes to use men as His helpers in the propagation of life, taught this when, instituting marriage in Paradise, He said to our first parents, and through them to all future spouses: "Increase and multiply, and fill the earth." As St. Augustine admirably deduces from the words of the holy Apostle Saint Paul to Timothy when he says: "The Apostle himself is therefore a witness that marriage is for the sake of generation: 'I wish,' he says, 'young girls to marry.' And, as if someone said to him, 'Why?,' he immediately adds: 'To bear children, to be mothers of families'."
Source: Casti Connubii (Vatican.va)