Since Roe vs. Wade, there have been three versions of the "Pastoral Plan for Pro-life Activities" issued by the US Catholic Bishops. The latest came out in November, 2001. In this third version, for the first time, the connection between abortion and contraception is explicitly discussed.
There are many aspects to this connection. First of all, some contraceptives cause abortions, and hence are not contraceptives at all. It is critical, moreover, to understand -- as the Pope points out in "The Gospel of Life" -- that abortion and contraception are specifically different evils that differ in nature and gravity. Abortion takes a human life; contraception distorts the meaning of human sexuality. Both are always morally wrong.
Contraception, strictly considered as preventing fertilization, is one of many factors leading to an increase of abortion in our world. As the bishops write, "…[S]ome promote widespread use of contraception as a means to reduce abortions and even criticize the Church for not accepting this approach. It is noteworthy that as acceptance and use of contraception have increased in our society, so have acceptance and use of abortion. Couples who unintentionally conceive a child while using contraception are far more likely to resort to abortion than others."
As Fr. Paul Marx, OSB, and I often discussed, there is no culture or subculture in the world that has permitted contraception and then has not gone on to permit abortion.
The ultimate root of the evil of contraception is that it denies that God is God. The attitude behind it is, "I am the one who ultimately decides whether a human being will come into the world."
As a result of that attitude, one thinks he or she can then change the meaning of sexual intimacy by holding back its life-giving power. Obviously, the same activity by which people express the deepest physical intimacy also can give rise to a new life. Did you ever wonder why God put these two aspects together in the same action? Could he not have invented one action to express love and intimacy, and another, separate action to bring about new life? Is it an accident that both belong to the same act, or did God run out of ideas?
Neither, of course. God acted with a deliberate and wise plan in creating human sexuality. His plan says that when one human being gives him/herself totally to another, that total "yes" includes a "yes" to new life. The partners put themselves in a stance of readiness. "Lord of my life and my body, in giving my body to another, I give my fertility, and I accept my partner's fertility. I am ready to accept your gift. Now I leave it up to you, my Lord, as to whether you will actually give that gift at this time."
As Dr. Bernard Nathanson explains, it is not that contraception causes abortion; rather, both are caused by the perversion of autonomy -- taking freedom and using it to stop rather than to welcome life.