First off two things: One I'm playing a devil's advocate and writing about why all forms artificial or natural of birth control are not with God's plan. I myself have not reached a definitive conclusion about the subject. Two, I welcome any comments to the for and against natural forms of birth control. That being said. Let the games begin.
There are many arguments for why NFP and any forms of birth control should not be used. I defer to http://www.mostholyfamilymonastery.com/Natural_Family_Planning.html
I will only add a few things to their arguments.
1 Corinthians 7:3-6
The husband should fulfill his duty toward his wife, and likewise the wife toward her husband. A wife does not have authority over her own body, but rather her husband, and similarly a husband does not have authority over his own body, but rather his wife. Do not deprive each other, except perhaps by mutual consent for a time, to be free for prayer, but then return to one another, so that Satan may not tempt you through your lack of self-control. This I say by way of concession, however, not as a command.
Okay. Saint Paul was an unmarried man. But in matters of faith and matrimony he felt that he could talk to people about how they should behave. He wanted them to yield to his feelings on the subject, but not feel that they are forced to. In this passage, he simply states that our bodies are not our own that we share each other as married couples. He said not to abstain from sex except during times of prayer. This does not mean to abstain from sex for times of regulation of births or spacing of births. He simply meant when devoting ourselves to God it is okay to abstain. He also said not to abstain too long because this can lead to infidelity and other immoral sexual acts.
Other parts of the Bible tell us to be "fruitful and multiply." The article I sited mentions some portions of this. But for Jews it was considered horrific to not bare children. Women were considered honored and privileged by God to do so. And those women who were barren would considered to not be blessed by God. The fact that Mary was a Virgin during ancient times was not deemed as a good thing. In fact the New Testament outlines the role of a good woman and what he felt widows should do.
1 Timothy 5: 14-15
So I would like younger widows to marry, have children, and manage a home, so as to give the adversary no pretext for maligning us.
Titus 2: 4-5
so that they (older widows who lead by example) may train younger women to love their husband and children, to be self-controlled, chaste, good homemakers, under the control of their husbands, so that the word of God may not be discredited.
In these passages, young women are told that in order to follow God they must be "self-controlled, chaste (meaning have fidelity towards their husbands), manage their home (good homemakers), marry, and yep....have children. It seems to me that it would be against God's will to practice not having children. There is I must note no information as to how many children or if it is even good for a woman to have children, but based on Jewish thought at the time. The more children the merrier.
In fact the Bible actually says that women are saved through motherhood.
1 Timothy 2:15
But she will be saved through motherhood, provided women persevere in faith and love and holiness with self-control.
Again this is another reason why it would be against God's will to prevent having children.
St. Augustine wrote: "Relations with one's wife, when conception is deliberately prevented, are as unlawful and impure as the conduct of Onan who was slain."
St. Thomas of Aquinas wrote: "Next to murder, by which an actually existent human being is destroyed, we rank this sin by which the generation of a human being is prevented."
These are the same arguments that many who practice NFP explain why artificial birth control is sinful and immoral. If you look at the two quotes, you will realize that both St. Augustine and St. Thomas of Aquinas were also referring to natural methods of birth control. Onan is from the Bible.
Onan, however, knew that the descendants would not be counted as his; so whenever he had relations with his brother's widow, he wasted his seed on the ground, to avoid contributing offspring for his brother. What he did greatly offended the Lord, and the Lord took his life too.
According to Jewish law, if a man died leaving behind no children it was the duty of his brother to marry the widow and continue the line. Onan used the natural means of the pull-out method to prevent pregnancy. This angered God. St. Augustine says that any means natural or artificial to prevent children is immoral.
St. Thomas Aquinas says that preventing children is akin to murder.
I again defer to the article's information about Pope Pius XI's remarks on birth control.
I would like to point out that Church contradicts itself when discussing NFP.
In contrast, "every action which, whether in anticipation of the conjugal act, or in its accomplishment, or in development of its natural consequences, proposes whether as an end or as a means, to render procreation impossible" is intrinsically evil.
This is simply stating that if foreplay gets out of hand or if you actually engage in sex and you render procreation (the creation of children) impossible that is immoral. To me this means conscientiously deciding to engage in sex at moments of infertility. Although some make argue this means using things like artificial birth control or the pull-out method because that is deliberate. Although I feel that by charting, taking your temperature, examining your cervical mucus, you are also deliberately render procreation impossible when refusing your spouse.
Further the Church states many times that it is the duty of married couples to have children and to raise them in the Church.
Called to give life, spouses share in the creative power and fatherhood of God. "Married couples should regard it as their proper mission to transmit human life and to educate their children."
The state may not legitimately usurp the initiative of spouses, who have the primary responsibility for the procreation and education of their children.
There is nothing in the Catechism that says that you should use NFP in order to prevent births. It does call for use of periods of abstinence for the regulation of births meaning spacing and number, but it explicitly state that it is the duty of the spouses to create life. One could argue that the distinction is blurry at best. You walk a fine line between preventing births for spacing reasons and preventing births altogether.
Because NFP is a form of birth control is it immoral? One could argue because you are not "actively" preventing pregnancy that it is not the same as artificial birth control. However, NFP is detecting periods of fertility and denying one's spouse sexual intimacy in order to prevent pregnancy. St. Paul clearly states that the only times to deny our spouse is during times of prayer. Furthermore, St. Thomas Aquinas says that preventing children is like committing murder. The RCC skates a fine line and leaves it in the hands of couples to decide whether they are practicing good morals by preventing pregnancy through abstinence for "good" reasons or "bad" reasons.
My husband makes the argument that the RCC took a middle ground approach to birth control. Feeling pressure to find a method to regulate births that could be fit in with the Bible's and scholars' teaching, they decided that NFP was acceptable because it left users "open to life." Although NFP brochures claim that NFP is 99 percent affective at preventing pregnancy when used properly. This is the same jargon used by manufacturers of artificial means of birth control.
But the question is not whether NFP leaves couples open to life, but rather is it moral to use any means to prevent pregnancy. And since NFP is a form of birth control, it does prevent pregnancy.