NFP and Intention

Generally, intention is knowable only to the penitent after examination of conscience, not by external criteria. The desire for pleasure is always at least a part of the intention; otherwise the act is impossible.

Traditionally, the only intention that could justify marriage was that of procreation, though some modern theologians hold that the unitive purpose of marriage might also suffice, as long as procreation is not opposed.

Even under the older, stricter standard, you do not have to be consciously trying to have a child each time, which would be an unrealistic expectation in any event. It suffices to have a general receptiveness to procreation, imposing no obstacle that would frustrate the natural fertility of the act. To use Thomist terminology, the intention can be actual or virtual. Actual is if your intellect is currently aware of it, and virtual is the continuing influence of a past intention of which you are not currently conscious. St. Augustine allowed that the intention to procreate can be virtual with his analogy of deciding to sleep for health, yet not thinking of health while sleeping. (Contra Iul., V, x, 42)

Contraception, whether using drugs, devices or perverse acts, is designed to frustrate the natural fertility of the act, so it is certainly incompatible with procreative desire. This would imply that the act is performed solely for pleasure, which is always at least a venial sin. In the case of contraception, it is a mortal sin, since you are taking positive action to destroy the fertility of the act. As St. Thomas says, it is no light sin, because it perverts an important natural good, the preservation of the species. "Wherefore, after the sin of murder, whereby human nature is deprived of actual existence, this kind of sin, whereby the generation of human nature is hindered, holds seemingly the second place." (Summa Contra Gentiles, I, cxxii)

NFP can be used with a variety of intentions, even to increase fertility. Applying the principles above, if it is used to temporarily reduce the chances of pregnancy for some serious cause, there must still be some actual or virtual intention to procreate for it to be without sin. Can it be said that the intention to procreate in any way influences present actions? This could be true if there are "well-grounded reasons for spacing births, arising from the physical or psychological condition of husband or wife, or from external circumstances," (Humanae Vitae, 16) so you are not necessarily trying to have few or no children, but trying to space apart their births, and ensure their good upbringing (which falls under the procreative desire). In other words, if your use of NFP is oriented toward the good of your children, rather than your own selfishness (i.e., it would be inconvenient to you to have more children), then there need not be any sin at all in NFP.

If the use of NFP is not motivated at all by the good of children, then it is at least venially sinful, insofar as the marital act is performed solely for pleasure (following the consensus of traditional theologians).

Misuse of NFP could conceivably rise to the level of mortal sin, though I'm not aware of any teachings defining the criteria for that. The mere absence of procreative intention would not suffice, since, unlike contraception, there is no intrinsically disordered act. The Church has never held that the use of marriage during an infertile period is itself sinful. As St. Thomas teaches, there is no no sin if generation is "accidentally" impeded by the fact that a wife is sterile (i.e., due to age or health). (Summa Contra Gentiles, I, cxxii) If natural procreation is frustrated by physical accident, this is not a moral fault. The mortal sin, if any, would not be in the use of marriage during infertile times, but in an evil intention to prevent procreation, without any regard for the good of children.

Again, I don't pretend to give any definitions where there is no clear teaching, but it would seem that there is a likelihood of mortal sin in cases where the spouses' intentions are no different from those who use artificial contraception: i.e., to utterly avoid bearing children at all costs, perhaps intending though not actually resorting to abortion in case of failure, or intending to continue to prevent conception indefinitely, or using the method without grave cause, but for mere convenience, showing a serious lack of esteem for the goal of procreation.

There are a couple of other unclear points. First, it is unclear if a marital act done more than minimally, though not solely, for pleasure is free from sin. Various doctors and theologians held a dim view of any desire for pleasure beyond that which was indispensable and subordinate to procreation, but this was never a universal opinion.

Also, it is unclear if a merely habitual intention to procreate suffices to justify a marital act. In other words, it might suffice to have a past desire to procreate, never retracted, yet nonetheless not influencing the current act. I make this conjecture only to account for the teaching that the continued use of marriage past the age of fertility is sinless. In such cases, it seems implausible that the spouses are motivated by any actual or virtual intention to procreate. Rather, they simply have retained the natural habits of their fertile age, and do not sin since this inclination originally arose from a natural intention to procreate, no longer active but never retracted. This seems to find support in St. Thomas' assertion that natural inclinations, even of brute animals, can never be toward that which is evil in itself, and this applies to the inclination to carnal intercourse. (Summa contra Gentiles, I, cxxvi)