The very first thing that comes to my mind when I hear that question is "The end does NOT justify the means." Not only is this saying common sense, but it is also a teaching in the Catechism istself.
1759 "An evil action cannot be justified by reference to a good intention" (cf. St. Thomas Aquinas, Dec. praec. 6). The end does not justify the means.
1760 A morally good act requires the goodness of its object, of its end, and of its circumstances together.
For those who still are wondering about the moral difference between contraception and NFP, the following is taken from a pamphlet simply titled Birth Control. IMPRIMATUR:Most Reverend Fabian W. Bruskewitz, D.D., S.T.D. Bishop of Lincoln
Contraception is the intentional use of a drug, chemical, device or procedure to prevent pregnancy by acting directly against the fertility of each marriage act (sex). The biological purpose of sex is to reproduce, yet contraception denies the goodness of fertility. It is a lie in "body language" (CCC 2370). It works against our nature, i.e. God’s purpose for creating us. Contraception is analogous to enjoying a delicious meal and then vomiting with the intention to lose weight yet satisfy the appetite (binge-purge gluttony). It also helps to promote the sins of adultery (sex outside of marriage) and fornication (sex before marriage) by reducing the chance of "embarrassing" consequences. Finally the more convenient forms tend to operate by inducing early abortion, e.g. IUD and RU-486. Even the Pill and Mini-Pill do cause early abortion by preventing "the acceptance of a fertilized egg in the womb." [The Pill Book, 8th (Bantam Books, 1998) p. 247; also see PDR 53 ed. (1999) p. 3326]. Contraception is a type of sexual "gluttony."
In NFP, couples do not work directly against the fertility of the marriage act but regulate birth by periodically abstaining from the marriage act. The act is periodically avoided and not abused. NFP only gives information to help a couple choose between abstinence or the marriage act. During the woman’s naturally infertile times, spouses can engage in the marriage act; whereas, during the fertile periods, they can abstain. Under NFP, "the married couple make legitimate use of a natural disposition;" whereas in contraception, "they impede the development of natural processes." [Humanae vitae 16] Even though the moral intention is to regulate birth by abstinence, NFP still respects the goodness of human fertility. It is analogous to fasting with the intention to lose weight. In similar fashion, fasting respects food as a gift; whereas, gluttony abuses food. NFP does demand "just reasons", self-discipline, sacrifice, mutual consent (1 Cor. 7:5; Casti connubii 53) and openness to new life (CCC 2366) from both spouses.
Unfortunately too many people do not understand freedom. For some, freedom is choosing the "easy way." But true freedom is rarely easy (John 8:32; 14:6). Following sexual urges without constraint is not freedom but giving in to impulse. Gratifying impulses eventually leads to "slavery" (e.g. addiction; Titus 3:3). As Jesus reminds us:
"...the gate is wide and the way is easy, that leads to destruction, and those who enter by it are many." [Matt. 7:13-14]
Having the freedom to climb a mountain or start a new business is not easy but demands sacrifice. Likewise continence in NFP may be difficult, but gives us, with God’s grace, the freedom to overcome sexual impulses and selfishness in order to pursue true love. Sense pleasures are good, but they are not the final purpose of human life (1 Cor. 10:31, Gal. 5:13).