There are a variety of resources for learning more about the reasoning behind the Catholic Church's moral teachings. The following might be a helpful starting point regarding NFP:
Birth Control and NFP: What's the Difference?
http://www.priestsforlife.org/library/4741-birth-control-and...
"NFP is not just a 'method' based on physiology. Rather, NFP is based on virtue. It is based on sexual self-control, which is necessary for a healthy marriage. There are times in any marriage when spouses have to put aside their desire for sex because of sickness, fatigue, travel, or other reasons. In a healthy marriage, love is shown in many ways, and not all these ways of showing love are physical. In fact, to refrain from sex when necessary is itself an act of love. Why? Because in effect the spouses then say to each other, 'I did not marry you just for sexual pleasure. I married you because I love you. You are a person, not an object. When I have sex with you, it is because I freely choose to show you my love, not because I need to satisfy an urge.' Using NFP requires abstinence from intercourse during the fertile days if a pregnancy has to be avoided. This actually can strengthen the couple's sexual life. When the spouses know that they can abstain for good reasons, they also come to trust each other more, and avoid the risk of treating each other primarily as objects of sexual pleasure rather than persons. Artificial birth control, on the other hand, gives free reign to the temptation to make pleasure the dominant element, rather than virtue. It encourages couples to think that sexual self-control is not necessary. It can encourage them to become slaves to pleasure."