Dear Fr. Matthew,
Here is my brief answer to your question. According to God’s plan for human love, we are always to be open to the good of our fertility, since it is directly related to the procreation of a unique person, made in the image and likeness of God. We should never turn against the good of our fertility. Thus every marital act should be open to potential parenthood, since love and life go together. When a couple has good reasons for spacing their children, then NFP is a morally acceptable way, because it always respects these values.
If the objective is to space births, and not become pregnant at this point in time, then the couple is to respect the requirements of their own biology. In short, if you choose not to become pregnant, then refrain from those actions that lead to pregnancy. And now for the longer answer. It is taken from the Couple to Couple League’s handbook in Spanish, Panificacion Natural de Familia, written by Erick Carrero. It deals with “Creative Abstinence,” and I am translating it for you.
“Many couples feel very bad when they discover that NFP requires abstinence from marital relations. They think that their spouse is going to lose interest in them. They do not know that abstinence can be an opportunity to improve their marriage. To abstain means to deprive oneself of something to which you have a right, with the purpose of obtaining something of greater value. When one abstains from sexual relations during the fertile time, one does so in search of a greater good. It could be for the health of the wife, for the husband’s search for employment, or time for taking care of the children which they already have. There are different reasons which each couple should place before God to decide if this is the appropriate way for them, or not.
“But this deals not only with limiting sexual relations. The time during which the couple abstains is the best time to cultivate other dimensions of their marriage. It is for this reason that we call it CREATIVE ABSTINENCE.
“If you recall when you first got acquainted, both of you shared many things about your lives and your feelings which had nothing to do with sex. This time is a special moment to be together, to prepare a special meal for your husband, to give flowers or to say something romantic or special to your wife. Also you can go on a walk, to the movies, to the park, speak about things that you desire to do together in the future, etc.
“Going to Church during the week, or making some special prayers together can be a real help in difficult days. Yes, difficult days because in many moments abstinence can be difficult. But they should remember that we deprive ourselves of something in order to obtain a greater good. When at last a couple can continue their relations, these tend to reflect a love more mature and more committed between the spouses. The husband will know that his wife wants him because she loves him, and not because of pressure or obligation. The wife will feel greater respect for a husband who is disposed to wait for the good of all.”
Fr. Matthew Habiger OSB