[Warning: This post contains what some might consider to be graphic descriptions of a sexual nature. I'm not being risque, it's just the nature of the subject.]
Previously I posted on whether Natural Family Planning (NFP) and artificial birth control (ABC) are morally the same; and whether NFP can logically be called “contraception”. I’ve continued to ruminate on the subject and came up with the following.
1. The argument that NFP is no better than ABC sometimes runs like this: The objective of both NFP and ABC is to have sex without conceiving children. Therefore they’re essentially the same thing. How can you allow one without allowing the other?
The unspoken major premise is that actions having the same objective are, essentially, morally the same. But isn’t that rather a shaky premise?
In fact, it follows the same reasoning as “the end justifies the means”, since the argument is basically: If the end of having sex without conceiving children is justified, then any and all means to achieve that end are equally justified. Thus if you allow NFP, you must also allow ABC. This line of argument seems clearly fallacious.
2. Something I mentioned in the comments, but which I would like to highlight here as well:
NFP consists of abstaining from sex during fertile periods, and having sex during infertile periods. The part of NFP that people consider questionable, or similar to ABC, is not so much abstaining — which is obviously not birth control — but choosing specifically to have sex during infertile periods. In this, they say, people manifest the same attitude and intention as they would in using ABC. But I think this is a matter of perspective.
Consider that before the discovery of NFP, married couples would have sex whenever. But if they wanted to avoid having additional children, they would have two choices: Either (1) abstain from sex entirely, or (2) use ABC. To a faithful Catholic, of course, number 2 was not an option, so there was only abstention.
NFP gave such persons an additional choice: Rather than abstaining from sex entirely, they could abstain only at certain times. They would still rely on abstinence to avoid conception, the same as before. Only they would have to abstain less often.
By no stretch can this be considered contraception, or anything like the same thing, essentially, as ABC. ABC relies not on abstinence, but on artificial physical barriers or chemicals designed to thwart bodily processes. These are precisely what have always been condemned by the Church from the time of the Fathers.
3. Christian tradition is unanimous in condemning ABC, throughout Christian history. And I don’t mean merely suggesting that it might not be totally copacetic, I mean repeated, clear and resounding condemnations.
Those condemnations, however, don’t apply to NFP, since the existence of fertile and infertile cycles was not discovered and scientifically verified (and then not very accurately) until around the turn of the 20th Century. Rather, they refer to the use of such things as “evil appliances”, “poisons” or drugs, and the withdrawal method: Things which either block the natural result of intercourse, or else involve conducting it in an unnatural manner, i.e. by interrupting it.
St. Thomas Aquinas writes the following:
 It is evident from this that every emission of semen, in such a way that generation cannot follow, is contrary to the good for man. And if this be done deliberately, it must be a sin. Now, I am speaking of a way from which, in itself, generation could not result: such would be any emission of semen apart from the natural union of male and female. For which reason, sins of this type are called contrary to nature. But, if by accident generation cannot result from the emission of semen, then this is not a reason for it being against nature, or a sin; as for instance, if the woman happens to be sterile.
Summa Contra Gentiles, Book III, Chapter 122
Here we have St. Thomas saying outright, that there is no sin in a man and wife having intercourse while the woman is sterile. Note that “by accident” doesn’t refer to an unintentional occurrence. He means rather that the woman being sterile is an accidental trait of hers, as opposed to an essential trait. It’s of the essence of a woman to be capable of reproducing, but the accidents of time and age can affect whether she is actually fertile in a given instance. There is nothing inherently unnatural or sinful in having intercourse with a woman when she happens to be infertile due to the natural processes of her body.
Pope Pius XI concurs when he writes,
Nor are those considered as acting against nature who in the married state use their right in the proper manner although on account of natural reasons either of time or of certain defects, new life cannot be brought forth. For in matrimony as well as in the use of the matrimonial rights there are also secondary ends, such as mutual aid, the cultivating of mutual love, and the quieting of concupiscence which husband and wife are not forbidden to consider so long as they are subordinated to the primary end and so long as the intrinsic nature of the act is preserved.
Pope Pius XI, Casti connubii (1930)