If a couple is simply using NFP as an effective method of birthcontrol so they can enjoy the marital embrace and avoid more children, aren’t they guilty of the same offense as a couple that uses contraception?
Sin is broken down into two parts: 1. the act itself; 2. the intention of the actor.
When it comes to part 1, NFP is not technically an action, it is the absence of an action (sexual activity during a fertile period). The procreative aspect of the marital act is considered an intrinsic good. While we should not act against an intrinsic good, we do are not obligated to promote it at every opportunity. For example, if I realize my brother-in-law is cheating on my sister Thanksgiving dinner in front of the entire family might not be the best time to reveal that truth. Remaining silent until a more appropriate time would be the right thing to do. Similarly, while the procreative aspect of the marital act is intrinsic to it we do not need to purposely promote it at all times. It is perfectly fine to refrain from the marital act for the right reasons.
When it comes to part 2, the intentions behind NFP could be as sinful as contraception. As the Catechism notes:
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.
While NFP itself is a lack of an action, our intention behind the lack of action could be sinful. Using the previous analogy, while remaining silent can be the right thing to do in certain circumstances, in other situations it can also be done with malice.