The teaching of the Catholic Church is that every conjugal act must be open to life. It follows that any method or act which seeks to make conception impossible is wrong. Some methods are worse than others because they are both contraceptive and potentially abortifacient. Natural Family Planning is a licit method -- because still open to life -- to plan a family when there are objectively sound reasons for doing so.
NFP is a natural contraceptive.
They are teaching you when you can have sex and not conceive a child without using a chemical or mechanical contraceptive.
Therefore no sex when the woman is already pregnant?
NFP vs Contraception
Spacing children may be a desirable goal that does not violate God's laws in certain serious/grave situations. But the means of achieving the goal differ.
One is intrinsically evil (abortion, abortifacient contraception, barrier methods, sterilization) while one is morally neutral (Natural Family Planning.
In one, an act is performed (sex) but its natural outcome is artificially foiled.
In the other, no act is performed (simple abstinence during fertile times) so there IS no act, therefore the practice is morally neutral.
It is then the intention of using NFP that constitutes its relative moral licitness or illicitness.
If NFP is used in a selfish manner, it too can be sinful.
If it is used only in grave circumstances, it is not sinful.
The difference is real.
Dieting (decreasing caloric intake, the "act" of NOT eating) is a moral and responsible means of losing weight to maintain the body's health.
Bulimia (the ACT of eating, them vomiting) is rightly called an eating DISORDER.
An ACT is performed (eating in this case) and its natural outcome (nutrition) is foiled by expelling the food from the body.
Likewise contraception is a disorder. An ACT is performed (sex) and its natural outcome (procreation) is foiled by expelling the sperm or egg or both (abortifacient contraceptives) from the body.
Contraception is to NFP what Bulimia is to dieting.
But just as dieting can be misused (anorexia) so too can NFP be misused in a sinful manner.
Finally, regarding infertility, if the sterility of the couple is through no act or conscious fault of their own, their marital relations cannot possibly be immoral, since it is not their intent to be sterile.
Yet many the Church married, who were thought for decades to be sterile, have indeed borne children in their later years. So it would be awefully presumptuous of any Church to disallow an "infertile" couple to marry.