Why do I think NFP is good and contraceptives are not? I definitely don't think it's "wrong" to use contraceptives. My husband and I have decided that it is wrong for us, but I can't speak for everyone else. I would say that it's not ideal for marriages, in general. I feel that, in general, couples should not be so closed to the idea to having children that they feel they must use contraceptives. I feel that NFP keeps couples thinking about children but provides a way to avoid pregnancy when approriate (like for me to finish college right now). Having used the Pill, condoms, and now NFP, I really think that NFP is the best. It makes us feel closer, like we're not holding anything back.
I said earlier that I believe the purpose of marriage is procreation and illustration (of the union between Christ and His Church). I think unity between spouses is included in illustration. Some, if not most, Christian denominations teach this. Procreation is not the "only" purpose of marriage, but, in general, it is a large part of it. I know there are couples who cannot have children. I believe those couples are called to do something else, whether that be adopt or devote their lives to something that would not be conducive to children. For example, the couple who runs the ministry my husband and I are a part of cannot have children. They believe it was a blessing in disguise because the work they do would be very challenging if they had children.
I understand that there are medical reasons for some couples to avoid pregnancy. Other than sterilization, NFP is as effective a way as any to do that. However, since I do not have experience with that, I don't feel justified in telling someone in that sort of situation to use NFP. Again, I don't believe it's "wrong" to use contraceptives. It's not a black and white issue.
There are a lot of books that do a much better job than me in explaining the reasons to use NFP. My husband and I did not base our decision to use NFP on something we read in the Bible. It is just a conviction for us.
I agree with the poster that said people sometimes overanalyze things. That was my point when I made the statement that there are many ways a marriage can fall short of the glory of God, and pre-marital sex is just one of them. I have friends and relatives who dwell on pre-marital sex way more than I think is necessary. I feel that couples should spend time thinking about sex within the context of family rather than "So how far can we go before marriage and it be okay?"
I believe that personal issues within your marriage are between you, your spouse, and God - not the rest of the world who will tell you a million different things. The Bible does't have all the answers to all our problems, only God does. If he gave us the solutions manual to life, how would we ever learn anything and why would we ever go to Him for help?