According to Catholic philosopher Elizabeth Anscombe:
For it was obvious that if a woman just happened to be in the physical state which such a contraceptive brings her into by art no theologian would have thought the fact, or the knowledge of it, or the use of the knowledge of it, straightaway made intercourse bad. Or, again, if a woman took an anovulant pill for a while to check dysmenorrhea no one would have thought this prohibited intercourse. So, clearly, it was the contraceptive intention that was bad, if contraceptive intercourse was: it is not that the sexual act in these circumstances is physically distorted. This had to be thought out, and it was thought out in the encyclical Humanae Vitae.
Here, however, people still feel intensely confused, because the intention where oral contraceptives are taken seems to be just the same as when intercourse is deliberately restricted to infertile periods. In one way this is true, and its truth is actually pointed out by Humanae Vitae, in a passage I will quote in a moment. But in another way it's not true.
The reason why people are confused about intention, and why they sometimes think there is no difference between contraceptive intercourse and the use of infertile times to avoid conception, is this: They don't notice the difference between "intention" when it means the intentionalness of the thing you're doing - that you're doing this on purpose - and when it means a further or accompanying intention with which you do the thing. For example, I make a table: that's an intentional action because I am doing just that on purpose. I have the further intention of, say, earning my living, doing my job by making the table. Contraceptive intercourse and intercourse using infertile times may be alike in respect of further intention, and these further intentions may be good, justified, excellent. This the Pope has noted. He sketched such a situation and said: "It cannot be denied that in both cases the married couple, for acceptable reasons," (for that's how he imagined the case) "are perfectly clear in their intention to avoid children and mean to secure that none will be born." This is a comment on the two things: contraceptive intercourse on the one hand and intercourse using infertile times on the other, for the sake of the limitation of the family.
But contraceptive intercourse is faulted, not on account of this further intention, but because of the kind of intentional action you are doing. The action is not left by you as the kind of act by which life is transmitted, but is purposely rendered infertile, and so changed to another sort of act altogether.
There's all the world of difference between this and the use of the "rhythm" method. For you use the rhythm method not just by having intercourse now, but by not having it next week, say; and not having it next week isn't something that does something to today's intercourse to turn it into an infertile act; today's intercourse is an ordinary act of intercourse, an ordinary marriage act.
I already commented on this once before, but now I'd like to make a more specific criticism of her argument:
i) A basic problem with her argument is that even if you accept her abstract distinction, there's no practical distinction in the case at hand. What motives a Protest couple to practice artificial birth control? (a) the desire to have conjugal relations; (b) the desire to avoid conception.
What motivates a Catholic couple to practice natural family planning? (a) the desire to have conjugal relations; (b) the desire to avoid conception.
At the level of intent, their intentions are identical in both cases, whether they practice artificial birth control or natural family planning.
ii) Now, it may be that Anscombe is using "intent" in the technical sense of double effect theory, where the agent did not intend the bad effect insofar as that was an incidental and undesirable effect of what he positively willed. But even if we accept that distinction, it fails to salvage Anscombe's argument:
a) Avoiding conception by exploiting a woman's infertile period isn't an undesirable side-effect of their action; rather, that's a primary motivation. They want to avoid conception. That's their direct intention. And successfully evading procreation is the desired result.
Moreover, their action is a means to that end (pace double effect theory).
iii) Furthermore, she switches arguments. She begins by distinguishing between different kinds of intent, but then shifts to different kinds of actions.