Let's start at the end and go up

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And what exactly is NFP?
NFP or (Natural Family Planning) is a technique, in line with the Church's teaching on contraception (we'll get to that in a sec), to space children
or to help conceive a child. The method uses a women's basal body temperature (and other physical sign, won't go into

) to determine when she is most likely to be fertile. It is 99.9% effective when practiced properly.
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As I understand it using condoms or other birth control measures is contrary to God's desire for us to "be fruitful and multiply." However, to use NFP is not sinful.
Yes, a better way to describe it is that God created the conjugal act to have two
inseparable functions within
marriage: procreation and the unity of the couple. Therefore, every conjugal act MUST have both of these components to be within God's will. When using NFP, one abstains during the fertile time thus avoiding pregnancy but still fulfilling the procreative and unitive aspects of the act becasue there is no
block to the pregnancy. The conjugal act is occurring during an infertile time of the month. Artificial contraception, however,
blocks the procreative aspect of the act thereby rendering it sinful. True love is always open to life and is fruitful. Sex is the ultimate expression of self-donative love between a couple. When the fruitfulness of the act is blocked - it becomes an act of pleaseure, only. Under these circumstances, the act becomes inherently selfish - and love is never selfish:
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Love is patient and kind; love is not jealous or boastful; it is not arrogant or rude. Love does not insist on its own way; it is not irritable or resentful; it does not rejoice at wrong, but rejoices in the right. Love bears all things, believes all things, hopes all things, endures all things. 1 Corinthians 13:4-7
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What I am wondering is how to distinguish the intentional "thwarting" of pregnancy through NFP from that of using condoms. Isn't the intention of both actions the same?
It is a difference of means and ends. NFP and contraception have the same end - avoiding pregnancy. BUT one employs licit means (NFP) and the other sinful means (contraception). Furthermore, NFP can be used illicitly when it is used to inordinately avoid pregnancy (say for selfish reasons.)
Good sources - The Magisterium of the Catholic Church. YOu can find its teaching in
Humanae Vitae and the
Catechism of the Catholic Church #2370. Really 2366-2372. (These numbers designate the paragraph (or article) numbers within the Catechism.)