Tribemike,
Youare absolutely correct that intentionally divorcing conjugal relations from the procreation of children is a sin. What you are not acknowledging is that when a couple uses NFP for legitimate reasons such as a grave health concern, they are in no way resisting God's ability and perogative to create life. They remain open to life in the act of marriage, but for serious reasons abstain during the days on which conception is most likely to occur. Couples using NFP take no action to prevent conception when they are intimate. NFP does not attempt to alter the unitive and procreative nature of sex; contraception works to make each sexual act infertile, deliberately eliminating its procreative potential.
While NFP can certainly be abused--and the Church is very clear that it should not be used for reasons of convenience or out of a desire to avoid children-- its mechanism of abstinence is conceptually and practically opposed to contraception.
Would you argue that chaste unmarried people are practicing birth control because their abstinence causes them to "avoid pregnancy?"