In Humanae vitae St. Paul VI reflects upon how God designed the nature of married love. This he draws from Sacred Scripture and the Catholic Tradition. The Holy Father writes that married love is fully human (involving the body and soul). Married love is total, faithful, and fruitful (see HV, 9).
"Fruitfulness" is discussed primarily in terms of God's gift of fertility to husband and wife. So important is this gift that Scripture teaches that God calls husband and wife to take up sacred stewardship over the powers of life that He shares with them (see Gen. 1:26-28; cf. HV, 8). Indeed, this gift is directly related to how God created men and women in the divine image (made for communion with God and each other - life-giving love). Children are gifts from God. They are part of the fabric of married love. Consequently, spouses should be open to receiving children lovingly seeking to nurture them as they form their families. Procreation and care of children go hand in hand!
Humanae vitae raises up the question of planning births in a family. The Church has always taught that it is reasonable for husband and wife to space and even limit births in their marriage for just reasons (see HV, 10); however, they are called to support God's design not to reject it. The use of contraception or sterilization rejects God's gifts. That is because contraception and sterilization do harm to the nature of married love and the gift of life - they separate the unitive and procreative nature of conjugal relations (see HV, 14). Saying this another way, "husband and wife express their committed love not only with words, but with the language of their bodies...the mutual gift of fertility is an integral part of the bonding power of marital intercourse." (USCCB, Married Love and the Gift of Life, 4, 5.) To reject one's fertility by using contraception or being sterilized is to reject God's gift to husband and wife (see HV, 12).
Does this mean that couples are to leave their family size to chance?
God invites husband and wife to cooperate with Him in their mission of responsible parenthood. Spouses should pay careful attention to the nature of marriage as God created it, their current responsibilities to each other, any children already born, and the wider society (see HV 10). Within this context, the methods of Natural Family Planning (NFP) are acceptable because they respect God's divine plan for marriage!
To deepen your understanding of these beautiful teachings see...