NFP vs. Contraception

"Isn’t NFP the same as contraception if a married couple is using it to postpone or avoid a pregnancy that they are not ready for?"

The short answer is "No." The reason is, contraception involves the deliberate frustration of the marriage act; NFP does not. In some ways, that may seem like a small difference, but in reality, the difference is huge and very important.

Traditionally, the Catholic Church has always taught that married couples have the right to "plan" their families, provided this is done in a responsible and just manner, and is done with the proper motivation. The Catechism of the Catholic Church says:

2368  A particular aspect of this responsibility concerns the regulation of births. For just reasons, spouses may wish to space the births of their children. It is their duty to make certain that their desire is not motivated by selfishness but is in conformity with the generosity appropriate to responsible parenthood. (emphasis in original text)

So, it is not "birth regulation" that the Church opposes, but selfishness and any immoral means of accomplishing that.

The Church teaches us that it was God who made us male and female, and therefore, it was by His design that the marriage act has the dual purposes of the procreation of offspring and the nurturing of love between the spouses. These purposes, designed into the marriage act, must always be respected in order to follow God's will. When a couple deliberately frustrates the procreative potential of the marriage act through contraception, they are acting against God's plan and design for marital love. On the other hand, when a couple who have a "just reason" for avoiding pregnancy choose instead to abstain from the marriage act during the fertile time of the cycle, they are not acting in violation of God’s design.

Abstaining from the marriage act does nothing to deliberately change the procreative potential of the marriage act because there is no act. Again, it is not a sin to postpone or avoid conception for a just reason, but how a couple postpones or avoids conception can be sinful or it can be virtuous.

Author Christopher West addresses the difference between contraception and NFP in his book, Good News About Sex & Marriage:

Suppose there were a religious person, a nonreligious person, and an antireligious person walking past a church. What might each do?

Let's say the religious person goes inside and prays, the nonreligious person walks by and does nothing, and the antireligious person goes inside the church and desecrates it. (I'm framing an analogy, of course, but these are reasonable behaviors to expect.) Which of these three persons did something that is always, under every circumstance, wrong? The last, of course.

Husbands and wives are called to be procreative. If they have a good reason to avoid pregnancy, they are free to be non-procreative. But it's a contradiction of the deepest essence of the sacrament of marriage to be anti-procreative.

To put it another way, the Church calls spouses to love each other as they were created in the image and likeness of God. When a couple contracepts, they are deliberately acting to change the way one or both of them were created; i.e., their actions are saying to God, "We don't like your design and have a better idea." When a couple uses NFP and abstains during the fertile time, they are respecting God's design and honoring it by waiting for the infertile time.