The first command in the Bible, in order of importance, is, "Thou shalt love the LORD thy God with all thy heart, with all thy soul, and with all thy strength." The first command in the Bible that you will find if you start reading at the beginning is, "Be fruitful, and multiply, and replenish the earth..." Since this is one of the very first things God told man it must be pretty important.
One of the implications of this command is that it is normative for people to get married. Furthermore, if God enables one to be prepared and able, it is better to marry young so there can be more years to have children. Thomas Malthus (one of the bad philosophers from the 1800's) said that famines, disease, and war were good for mankind because they kept our numbers low. He also taught that couples should marry older so they would have less children. In contrast to this, the Bible speaks of the wife of one's youth and Psalm 127 speaks of 'children of the youth.'
Just as God blessed man telling him to be fruitful God wants all to come to repentance and become sons of God. God told the apostles to preach the gospel to every creature. They could not tell all the world by themselves but as those they taught teach others the Church can multiply also.