”Union and Procreation
12. This particular doctrine, often expounded by the magisterium of the Church, is based on the inseparable connection, established by God, which man on his own initiative may not break, between the unitive significance and the procreative significance which are both inherent to the marriage act.
The reason is that the fundamental nature of the marriage act, while uniting husband and wife in the closest intimacy, also renders them capable of generating new life—and this as a result of laws written into the actual nature of man and of woman. And if each of these essential qualities, the unitive and the procreative, is preserved, the use of marriage fully retains its sense of true mutual love and its ordination to the supreme responsibility of parenthood to which man is called. We believe that our contemporaries are particularly capable of seeing that this teaching is in harmony with human reason.”
Sex is ordered toward union and procreation. Or as Dr. Janet Smith would say, sex is for babies and bonding. This is how God ordained it. This is His eternal plan, and man on his own initiative cannot break this inseparable bond. Contraception, sterilization, sex outside of marriage, and homosexual acts are all contrary to God’s divine plan.
I think the last line was thrown in there to point out that this is common sense stuff. It’s not merely a “religious” topic. If you have sex, it is natural for the woman to get pregnant. Pregnancy and fertility are not diseases that need to be treated. Children are meant to be born within families where there is a father and a mother. And Paul VI reminds us that “this teaching is in harmony with human reason.”