Let me focus on artificial contraception.
Based on the natural law, sex is God’s means—using human beings as instruments—to create life. God desires to create human life and he designed the human body for the fulfillment of his will to create human life. To interfere with the natural process of the body to procreate through contraception or masturbation is to thwart the desire and design of God to create human life. No doubt God slew Onan. Sin is essentially going against the will of God. If artificial contraception is not going against the will of God to create human life, and therefore a sin, then I do not know what is.
The very fact that it is artificial means that it is against nature and the natural law, and the very fact that it is a contraception means that it is against conception and the will of God to create life. Evil is evil, whether we know/like it or not, and couples are guilty of a greater sin if they obstinately persist in practicing it after having been informed of its objective and intrinsic evil.
NFP fosters the natural function of sex, which is for procreation, precisely because it is natural and there is no artificial interference introduced into the body that would frustrate procreation. It is also open to the possibility of procreation because God could always step into the process and make conception possible, which must be welcomed by the couple as a gift of God and not disdained as an unwanted pregnancy. Hence, the method and the motive of the couple should not frustrate the function of sex for the transmission of life. NFP is perfectly within the natural process and does not do anything that would interfere with that process and therefore retains its intrinsic relationship to the procreation of human life.
On the other hand, in artificial contraception, the method and the motive of the couple ensures or is supposed to ensure the impossibility of transmitting life, especially when sexual contact is done during the time when the woman is fertile, which is sinful simply because the creation of human life is willed by God (cf. Gen 1:28). In other words, the substantial disparity is based on the fact that one is natural leading to non-conception, while the other is unnatural leading to anti-conception.
NFP, which constitutes both marital continence and coition, for achieving or avoiding pregnancy, is not natural contraception, which is a contradiction of terms. Contraception cannot be caused by the natural method of birth regulation. Abstention during the wife’s fertile period cannot cause contraception because there is no sexual intercourse in the first place. Likewise, sexual contact during the wife’s infertile period cannot cause contraception because her natural condition is not conducive to conception in the first place. Hence, there could be no contraception when marital continence or coition is carried out within ambit of the natural order of things ordained by God since there would be no conception to contradict.
Abstinence is the key to NFP. Of course, it does not follow that those couples who do it artificially cannot practice abstinence. The point is that marital celibacy, which is actually sexual abstinence, is better expressed, as subjugating one’s passions for a higher good or justifiable reason, when the couples refrain from sexual contact, especially when the woman is fertile. If the couple wants to express their love through sexual union then they should do it! But, they should not interfere with the natural course of things or shun the consequences. Otherwise, they should abstain. Moreover, marital celibacy is considered the greater good than artificial contraception or sexual intercourse that would kill the mother and/or the child because in this case, the couple is called to a higher vocation for the sake of the Kingdom of God (cf. I Cor. 7:29).
Reason enlightened by revelation understands that when married couples have sex, they are instruments of God in the creation of life. They facilitate the transmission of life with the aid of God or they aid God in his desire to create life (cf. Gen. 4:1). The couple does not create life, they procreate life; that is, they cooperate with God in the creation of life. We mirror God in all the things that we do because we are made in his image (cf. Gen. 1:26), and all the more in the act of procreation, which is a sacred act because God is involved in the process and that is why it cannot be done for recreation but only in the context of life and love. This love is also unitive because it brings couples together and unifies their persons, which is symbolized by their conjugal union. This is the design of God for man and woman in sexual love, which he blessed by instituting the Sacrament of Marriage (cf. Gen. 2:24) to sanctify human sexual activity because of its function as the instrument for the creation of human life.