Here is the text of a post I did for NFP vs. contraception, hope it helps.
All truth springs from God, and so the truth is to be found there. God is a Trinity of Persons all sharing one divine nature. For brevity sake I will not elaborate on that at the moment but if you would like I very good description of that concept which I lifted from Frank Sheed. Anyway, God the Father is infinite and exists outside of time all in one act of being. In that one act of being God the Father’s “knowledge of himself” can be thought of as the “thought of Himself” which is infinite and complete, so complete it is the Second Person of the Trinity, but possessing the same singular divine nature. What can you call a thought? When it is expressed it is word, the “thought (or knowledge)” therefore of God is the Word of God; Jesus Christ. Now in this same instant act of being outside of time God the Father and God the Son (the Word) give completely of themselves to each other – complete self giving love. This love is so complete, so giving and so infinite in it’s completeness it is another person, in fact it is the Holy Spirit. God’s love is in essence creative and God is love, His love gives rise to life and the Three Persons of the Holy Trinity are in an eternal state of mutual self giving love – God is a family of Persons, that’s why we have family covenants throughout Scripture. We are created in God’s image are we not? What happens when couples come together to give self-giving love to each other? Well, nine months later you might have to give that love a name! That is just one way we are made in God’s image and model God’s own interior life and participate in His creative act. When we use contraception we are specifically saying to God, “I want the love part, but I’m not open to the creative part.” We are specifically putting up a barrier to God’s will. In so doing we are choosing our self-will over the will of God, which of course is a sin and separates us from God. Natural Family Planning is different. In NPF we are not putting up a barrier to God’s will for each time the couple does come together in self-giving love they are open to God creating life in them if He so chooses. They may abstain during fertile times for fasting from sex is approved by Paul. So to summarize one is putting up a specific barrier to God’s will while the other is saying “God I’m not sure if now is the right time for children but we respect your divine will and providence and if you wish for us to become pregnant are open to it if you desire it for us.”
By the way, this also explains why homosexual acts and gay marriage are immoral and contrary to God’s Divine nature.