Zippy posted this and I am reposting it in its whole for my records so I can access it when I get into yet another NFP/ABC-are-morally-equivalent discussion. His words follow:
We will remain forever confused about the morality of acts as long as we fail to acknowledge the difference between doing this particular thing, and not doing some thing. Something done is a concrete behavior chosen, a potentiality actualized: a real part of the world. Something not done is an abstraction, a potentiality not realized: it is not something real. “I did X” stated by a corporal human being expresses a fundamentally different kind of (de)ontological truth than “I did not do X”. The latter can be evil in the presence of a positive duty to act; but it is always a mistake to confuse a positive concrete act with refraining to act. It is for this reason that the negative moral precepts prohibiting certain concrete behaviors or specific acts apply always and everywhere, whereas positive duties to act always fall under a prudential judgment. As Pope John Paul II tells us in Veritatis Splendour:
The negative precepts of the natural law are universally valid. They oblige each and every individual, always and in every circumstance. It is a matter of prohibitions which forbid a given action semper et pro semper, without exception, because the choice of this kind of behaviour is in no case compatible with the goodness of the will of the acting person, with his vocation to life with God and to communion with his neighbour. It is prohibited — to everyone and in every case — to violate these precepts. They oblige everyone, regardless of the cost, never to offend in anyone, beginning with oneself, the personal dignity common to all.
On the other hand, the fact that only the negative commandments oblige always and under all circumstances does not mean that in the moral life prohibitions are more important than the obligation to do good indicated by the positive commandments. The reason is this: the commandment of love of God and neighbour does not have in its dynamic any higher limit, but it does have a lower limit, beneath which the commandment is broken. Furthermore, what must be done in any given situation depends on the circumstances, not all of which can be foreseen; on the other hand there are kinds of behaviour which can never, in any situation, be a proper response — a response which is in conformity with the dignity of the person. Finally, it is always possible that man, as the result of coercion or other circumstances, can be hindered from doing certain good actions; but he can never be hindered from not doing certain actions, especially if he is prepared to die rather than to do evil.
Now, anyone who has followed my writing for long enough knows that I have my doubts about the rather loose attitude many Catholics seem to take with respect to NFP; at the same time, I acknowledge that the Magisterium affirms some reasonable moral latitude for its licit use.
The question of why NFP can in some circumstances be morally licit, while contracepted sexual acts can never be morally licit, can be understood by apprehending the difference between the negative prohibitions against certain concrete behaviors, on the one hand, and positive obligations to act, on the other. I don’t think it can be understood without reference to this key distinction, which JPII emphasizes in the encyclical. Once this key distinction is understood and embraced, however, the difference becomes clear.
A contracepted sexual act combines in one and the same behavior the pursuit of physical sexual satisfaction and a rejection of children. As an actual chosen behavior this is not merely an intention or disposition; this is an actual concrete real act in the world, an act which cannot be chosen without expressing hatred of children. Thus it violates a negative prohibition of the moral law: it is simply not possible for human beings with human nature to choose this kind of behavior with a good will, no matter what protests are made to the contrary. If that behavior is knowingly chosen it is done with an evil will, because that is the nature of the behavior and of the human being who knowingly chooses it.
Abstinence, though, is not a concrete real act or chosen behavior: it is the absence of a behavior, a potentiality which the acting subject chooses not to realize. In order for abstinence to be evil, therefore, there must be a particular positive duty to act at a particular place and time. But when it comes to the marital act, there is no positive duty for a couple to engage in it at particular places and times, where if they choose not to engage in it that choice not to do so is evil. If the couple decides not to get it on on the kitchen floor right here and now, that decision does not inherently by its very nature express hatred of children; else every moment the couple was not engaged in the marital act would be an expression of hatred of children. So while it is indeed possible, because of circumstances or intentions, for abstinence to be morally wrong, there is significant moral latitude in deciding not to act; whereas a contractepted sexual act, choosing to act in a particular way which by its nature expresses a hatred of children, is always morally wrong.