Your premise is not complete. The marital has three purposes, according to St. Thomas Aquinas. These three puposes are not of equal importance, but all must be present for the complete good of the act.
(St. Thomas defines evil not as a thing itself but rather a lack of good. That is critical to understanding this entire issue.)
The premise is that the marital takes place only in the bonds of a sacramental marriage. Here are the purposes in their order of importance.
- Procreation.
- Increase and re-enforcement of marital unity and the marital bond. It requires a complete and unconditional surrender of each spouse to the other.
- Legitimate self-donating pleasure. That is you give more than you expect to receive.
All sex acts must meet all of these purposes or the good is lost. Remember, the loss of good is evil.
NFP is used both to help couples conceive and to reduce the chances of conception. NFP used for either purpose does not inherently violate any of these three points as there is always the possibility for life. Now the rest remains with the attitude of the married couple.
NFP
can be illicitly when the wrong intentions are employed, for example, the absolute exclusion of children would be an abuse of NFP.
One more thing. I cannot understand how being abstinant is not excercising self-control. Can you explain that for me.
Finally. I will say that while I am absolutely conviced that the Church’s teaching is correct here, I** must admit that I am not very good at explaining it. ** I would highly recommend a book Theology of the Body for Beginners by Christopher West. It covers this and all the issues regarding the Catholic teachings on human sexuality, the proper relationships between men and women and the God-given roles, gifts, and responsibilities of each gender.