Now to the point of the discussion, the intent and motive, as you pointed out, are crucial to one’s culpability of mortal sin. The man who’s wife is secretly contracepting is participating in an act of contraceptive sexual union, no? The only thing that “saves” him from it being a mortal act is that he is participating in it without the intent or the motive to prevent pregnancy. He is deliberately participating in that act, however.
The couple who are engaged in marital union without the intent or motive to prevent pregnancy also are participating in a deliberate act, one of marital intercourse. So the same thing that “saves” the man (who’s wife is secretely contracepting) doesn’t apply to the could who’s intent is to be open to life, despite the fact that medication necessary to maintain their (one of them, at least) health prevents conception?
I’ll try to be more specific. You can go to my previous post for the definitions of “deliberate” and “intent.” But one has to be careful with these theological terms and to what “intent” is applied. It is not just “intent” that saves the man from participating in his wife’s sneaky contraception. It’s the fact that he is not deliberately consenting to the use of contraception with his will because he does not know of it. Intent is the movement of the will towards the end, or goal, so if he doesn’t know his wife is contracepting, he cannot have the intent to contracept, nor the will to contracept. But when you say he is “deliberately participating in the act” you are referring to the act of conjugal relations (sex), not to the act of contracepting. This is where the confusion arises. These are different acts, and they must be separated. They are two different acts with different intents.
The act of having conjugal relations was willful and a deliberate act. The act of contracepting was deliberate on the part of the wife because she intended it and willed it, but not on the part of the unknowing husband. The husband did not (could not) deliberately contracept by participating in contraception. However, his intent in the conjugal act (a separate act) is unknown. Perhaps they are married, living comfortably, and can afford to have another child. Perhaps the husband is selfish and self-gratifying and only having sex during infertile periods. Even though he does not act deliberately with regard to the wife’s contraception, he may have a separate contraceptive intent with regard to his sex life and this would render his act sinful, too. So, the husband may, or may not, be engaging in contraception (through his intent), separate from the wife’s actual contraception. Intent is subjective and is connected with, but sometimes separate, from a deliberate action which is freely willed. And there are two different acts, here.
The principle of the double effect applies, but not to the sexual act. This is the entire point. HV15 does not say anything about having sexual relations. It only says something about therapeutic means which may impair procreation. Moreover, it is no surprise. If this paragraph were not in HV, would it be permissible to use contraception to cure a person, even if made the person sterile? Of course. It’s the Natural Law, that you can try to heal a person, even if unintended (lesser) effects occur. It’s not true because the Pope wrote it; the Pope wrote it because it’s true. So, if you’re waiting for an explicit papal pronouncement about whether one can have sex while one is temporarily sterilized, then you’re placing quite a burden on the papacy. Draw your own conclusions. If you understand the principles of HV, the Natural Law, and moral theology, the conclusion is compelled.
The double effect means that there is a good effect, and a bad effect, but on balance, the good outweighs the bad. So, a terminally ill cancer patient needs morphine to stop the pain, but the morphine will hasten death. Well, death will occur anyway, so the good effect of using the morphine (pain relief) outweighs the bad effect of a hastened death. If the person were NOT terminally ill, and the pain relief caused death, this would be murder.
To apply it to the present case, if an operation is necessary to remove a cancerous uterus, that good effect would outweigh the fact that reproduction will be permanently impaired (bad effect). This has NO application to whether or not the couple may then engage in the conjugal act. The double effect concerns the cure (good) and the after effects (bad).
So, the “intent” of the “contraception” is not at issue. The therapeutic means is intended to cure, and its good effects outweigh the bad. But this does not concern the “intent” of the conjugal act. The fact remains that the person is sterile. Can she have sex?
Drawing upon the principles of the RCC, if the sterility is permanent then, according to the Natural Law, the person will never be able to conceive. So, she is no different position than a post-menopausal woman. She may have sexual relations.
If the sterility is temporary, then it is being temporarily “masked” by the medication. This means that a woman capable of conception, but for the therapeutic means, is rendered temporarily sterile. This is contraception. Suppose the woman is on the pill (to cure a cyst, for instance). She has contracepted. The fact that she does not intend contraception, and that the medication was not intended to contracept, does not exempt her. She has rendered her body temporarily sterile, and sins if she has conjugal relations. The double effect permits the treatment, but does not permit sexual relations. This is entirely consistent with the Natural Law, HV, and the fact that contraception is “intrinsically evil.” It’s not evil by “intent” (though that could make it evil, too). It’s evil by act - same as adultery, stealing, masturbation, etc.