'Be Fruitful and Multiply'
Children are indeed a blessing, and they are one of the Lord's purposes for creation. God made humans in two sexes, designed to complement one another, to image his glory, and by reproduction to fill the Earth with that glory. Amid the sexual chaos of our society, Christians need to be faithful to God's purposes for us as sexual beings created in his image.
The Bible also teaches that our sexuality can be properly fulfilled only in the secure garden of delights we call marriage (though sin disturbs it). Outside the bounds of marriage, sex is like luscious fruit that God has said is not for us—it does damage instead of good. Biblical commands against sex outside of marriage and divorce are designed to protect not only adults but also the next generation, the children who need a secure world of committed love and affection.
For Jesus, marriage is rooted in God's purposes at the Creation. The other side of the coin is that Jesus calls divorce and remarriage adultery, because it goes against God's created order for the male and female (Matt. 19:1-12; Mk. 10:1-12).
Following this teaching on marriage, Jesus blessed the children and warned adults not to stand in the way of the children coming to him. The implication is clear: Divorce and adultery create obstacles that can hinder children from entering the Kingdom. God's grace can and does overcome those obstacles, though rarely is the damage to children completely undone in this life. The opposite implication is also clear: Marriage ought to be a place where children experience God's promises and love in the kept promises and sacrificial love of their parents. In faithful marriage, children also experience a place where Christ's forgiveness heals and repairs the damage sin inevitably does. They discover that marriage is both blessing and work, and never one without the other.
The Truth of Genesis 1:28
Many Christians correctly oppose the sexual and marital chaos that has infiltrated our churches. But in this struggle against sin and for marriage and family, some Christian traditions take a wrong turn. They argue on the basis of the created order (sometimes called natural law) and Scripture that God has actually commanded married people to have children.
These Christians, who see this command as absolute, argue against birth control, except for what they consider the natural means of abstinence. They claim Genesis 1:28—"Be fruitful, and multiply, and fill the earth"—as a proof text. Birth control seems to disobey this commandment, which is rooted in God's purpose for creation.
Much could be said in response, but only one comment is essential: Genesis 1:28 is not a commandment, but a blessing. It does not refer to what humans must do to please God, but to what God does for and through humankind. The text says, "God blessed them, and God said to them, 'Be fruitful and multiply'" (RSV). Fertility is not a command but a blessing that God gives to his creatures, to animals as well as humans (Gen. 1:22). The filling of the Earth is a gift of God's wisdom and shows forth his glory as Creator (Ps. 104:24, 31; Isa. 6:3).