The Pope referred to the famous case of the 'Nuns in the Congo' in the latest aeroplane interview. The case is about nuns who, fearing rape, take some kind of contraceptive pill. Pope Francis' exact purpose in making the reference was unclear, but not nearly unclear enough for the Vatican spokesman Fr Lombardi, who relived his triumphs in obscuring the teaching of Pope Benedict XVI on the dangers of condoms for people with AIDS, and in throwing sand into the eyes of everyone trying to make sense of Pope Benedict's remarks about male prostitutes using condoms.
In the meantime, Sandro Magister seems to have uncovered the history of the 'Nuns in the Congo' discussion, which wasn't what pretty well everyone had assumed up to now, claiming that Pope Paul VI said nothing on the subject. Rather, it had simply been discussed by some theologians under Pope John XXIII.
Being a moral philosopher rather than a historian or, for that matter, a mind-reader, I think the contribution I can best make here is to explain why the Nuns in the Congo case is important, regardless of whether Pope Paul VI or any other pope authorised any ruling about it.
It should be obvious that the non-contraceptive use of devices or chemicals designed with contraception in mind is not necessarily wrong. Blowing condoms into balloons; using the Pill to control menstruation, and so on. Condoms are not intrinsically evil; it depends on what you do with them. What the Magisterium has also taught, for a long time, is that doing or omitting certain actions with the intention that conception will not take place, is not necessarily wrong either. If a couple don't think it prudent to conceive at a given moment, and choose accordingly to abstain from the marital act, this is permissible (assuming they have good reasons for doing this: I'm going to ignore this issue from here on, but have discussed it here).
What is wrong is (Pius XI) is the 'frustration of the natural act' with regard to its procreative potential, or to 'deprive it [sc. the marriage act] of its natural force and power'. (Casti conubii 1930)
Paul VI needed to emphasise that a pill taken hours or days before or after the sexual act was still wrong: it didn't need to make a difference, like a condom, to the act considered as physical behaviour. So he put it slightly differently: he condemned 'any action which either before, at the moment of, or after sexual intercourse, is specifically intended to prevent procreation—whether as an end or as a means.' (Humane vitae (1968) 14)
Both encyclicals make it clear that abstinence with the intention of not begetting children, even when the abstinence is targetted at moments when the woman is fertile, does not necessarily contravene the moral law. This of course is what has led to the development of more accurate methods of determining fertility with a view to 'Natural Family Planning' (NFP). You don't have to be an enthusiast for NFP, however, to see that any other ruling by these Popes would have been impossible. It would be absurd to say that couples are obliged to engage in the marital act when there is a war, plague, or famine raging and they are concerned about what will happen to the baby.
This means that an intention not to have a baby is not intrinsically immoral. What is intrinsically immoral is this intention coupled with the intention to engage in a sexual act (as opposed to not engage in such an act). To clarify, a couple using NFP will not engage in the marital act with the intention of not conceiving. That intention wouldn't make sense, because they have not done anything in relation to that act which will impede its leading to conception. Rather, the acts which they perform with an intention not to conceive are, in fact, ommissions to engage in the marital act at this or that time. There is no marital act whose 'natural force and power' towards procreation has been deliberatly frustrated by the couple; it is just that the potential marital acts which would have the most efficacious 'force and power' don't take place at all.
But if what is intrinsically wrong is the combination of these two intentions, or, as Pius XI describes it, to 'frustrate the natural act', then not only is abstinence permissible, but so is the use of contraception, even with the intention that it prevent conception, if there is no intention to engage in a sexual act. This would normally be nonsensical, but it could be at issue with cases of rape.
A big caveat is needed at this point, that the contraceptive method at issue must be contraceptive in the strict sense. If there is a danger that it will prevent the development or implantation of a fertilised ovum then it is a very different story, so I don't think this reasoning can be applied to the 'Morning After Pill,' and it doesn't look like it could be applied to the conventional Pill either. But there are many ways one can try to frustrate conception, and in principle this would be morally licit other things being equal.
In fact this conclusion was reached by Catholic ethicists long before Humanae vitae, and even before John XXIII. It is a commonplace of the old theological manuals that a victim of rape could, with the intention of frustrating conception, wash out the rapist's seed. This would not be permissible (at least, not with that intention), where the sexual act had been consensual, that is, intended by the woman.
This is all very technical stuff. I put it out here not because it sheds any light on what Pope Francis said on that aeroplane (long may it rust), but because in their frustration many Catholic commentators are making a great deal out of the fact that the 'Nuns in the Congo' case has never been authoritatively taught. This may well be true, but the theological consensus about the case is not a reflection of modernist corruption; nor yet is it an opening towards more exceptions and a hollowing out of the teaching on contraception. It follows from the moral principles which make up the teaching on contraception. The denial of the need for there to be an intention to engage in a sexual act, as well as an intention to prevent conception, to make up the intrinsically immoral 'contraceptive intention', would lead not just to pastorally inconvenient consequences, but morally absurd ones.
Related posts: questions about NFP here and here; on Pope Benedict's views on the use of condoms by (male?) prostitutes, here; on whether condomistic intercourse is always wrong here, and here.