The difference is very simple.
NFP involves the use of abstinence. It forces the couple to control and curb their sexual appetite, the lust of which along with gluttony and hatred is at the root of the concupiscence of original sin. It is always good for a Christian couple to abstain from sexual intercourse every so often to devote themselves more to prayer. If we do not curb our sexual appetite, our mind will become overwhelmed with thoughts of lust to the exclusion of God.
Artifical contraception involves the use of various measures to thwart the proper working of the human body. Its purpose is to allow full throttle indulgment in the sexual appetite.
Can you see the difference? Its not the end, but the means.
There is nothing per se wrong with the end of being married and not wanting a child right now, provided one does want children, and there in fact may be very good reasons for it because of finances or medical conditions. But the proper way of attaining that end is abstinence.
Sitetests comparison of artificial birth control to bullimia is spot on. These are identical disorders.
Hermann: "It is always good for a Christian couple to abstain from sexual intercourse every so often to devote themselves more to prayer. If we do not curb our sexual appetite, our mind will become overwhelmed with thoughts of lust to the exclusion of God."
Well, abstinance may be beneficial at times, but I still think there is concern about someone purposefully thwarting the intentions of God. It is God who opens and closes the womb, is it not??
NFP is contraception, whether you want to admit it or not - it is the intentional action of limiting sex to guarantee that conception cannot occur. You are still enjoying sex (during certain cycles) without the potential to conceive. The primary question is whether God commends this practice of contraception.
What does God say about this practice? One of God's primary commands to Adam was to be fruitful and multiply. Is NFP in line with this commandment? God has commanded His people to take dominion over the earth. Is NFP in line with this commandment?
I think you should seriously examine your actions if you are practicing NFP (or any contraception). Why are you trying to limit your number of children? Do you lack faith that God will provide for you/them? Are they an inconvenience? Do you not think that God will not bless those that obey His commands?
This is just another issue where our culture (I can't afford another child but I can afford that SUV) has corrupted God's precious and holy will.