Jesus said, "What God has joined together, let not man put asunder." In context, of course, He was talking about divorce. However, the idea is equally applicable to contraception. God "joined together" sex and conception. For man to separate them, either by having sex while intentionally preventing conception, or by conceiving children outside the natural process, is a violation of God's design for humanity.
In contrast, the use of NFP to avoid pregnancy does not separate sex and conception. It simply avoids both, for a particular time. However, it's important to remember that the Church says that abstinence may be used to avoid conception only for "grave reasons" or "serious motives." Obviously, this is a very subjective question, but it's one that every couple is required to consider with real generosity. None of us can judge what is "serious" for another couple, but we can be sure that God is calling all of toward a conscientous openness to life, and not merely an objective acceptance of conception "if we have a surprise."
Before I go on I want to you undestand I am NOT trying to pick a fight. I'm just a person struggling mightily with this teaching. Allright then......
"In contrast, the use of NFP to avoid pregnancy does not separate sex and conception. It simply avoids both, for a particular time."
That is untrue! NFP allows couples to engage on a very calculating method....the pleasure of love-making WITHOUT the probability of conception. Yes, I know there is a time where there is a cease and desist. But there is also a time where the pleasure of love-making is engaged WITHOUT the chance of conception. And it is here that I struggle mightily with the Catholic teaching. There are couples who engage in sex while practicing birth-control and become pregnant and go on to have their children accordingly. NFP'ers brag about the effectiveness of their method. Think about that for a moment.....NFP practitioners brag about the effectiveness of their method over and above the "other" forbidden methods. They are essentially bragging about the ability to NOT BRING LIFE INTO THE WORLD!
Actually, the words used by the Sacred Penitentiary in the 19th century under Bl. Pius IX and Leo XIII, and by recent teachings of Paul VI and John Paul II in the 20th century use the words "just reasons" (see the Catechism, #2346). The words: "grave reasons" appears only in Pius XII's allocution.