There is more than one way to assert a heresy. The simplest way is to directly contradict an infallible magisterial teaching. But another way is to substantially distort an infallible magisterial teaching. That latter form of heresy is what I’m discussing in this article. Dr. Ed Peters adheres to and publicly asserts material heresy on the infallible teaching which condemns contraception. He admits that this teaching is infallible, and yet he severely distorts that teaching.
In the post Misunderstanding the (alleged) ‘Congo contraception’ case, Dr. Peters asserts:
“Moreover, Church teaching on the immorality of contracepted marital acts is, I believe, taught infallibly; but, even if I were wrong about that technical claim, there is no question about what that teaching is, namely, that contracepting acts of marital intercourse, whether doing so as an end in itself or as means to some other end, is objectively immoral.
“A discussion could be had, I think, on whether non-marital sexual intercourse is subject to the same moral requirements as that to which marital intercourse is held. Humanae vitae does not, as far as I can see, address that question.” 
Peters asserts that he holds a particular idea to be infallible, that “contracepting acts of marital intercourse is objectively immoral.” Well, it is immoral for married persons to contracept. But then he goes on to narrow the infallible teaching on contraception, claiming that Humanae Vitae does not condemn the use of contraception in non-marital sexual intercourse. He also makes the broader assertion, not limited to Humanae Vitae, but on the subject of Church teaching generally: “there is no question about what that teaching is,” specifically, that contraception is immoral when it pertains to marital intercourse only.
The heretical distortion here is to limit the condemnation of an intrinsically evil and gravely immoral act to a substantially smaller subset of acts. And this limitation of a dogmatic teaching is particularly ironic given Peters’ repeated insistence that the divorced and remarried cannot receive Communion, due to their sexual acts within a non-valid marriage. Essentially, he is saying: “You can’t receive Communion, due to intrinsically evil and gravely immoral acts of adultery, but your use of contraception is not condemned by the Church.” Of course, the use of contraception is not a public or “manifest” sin, but taking Canon 915 and 916 together, any grave sin should generally prohibit from Communion. So he is rebuking the divorced and remarried for one grave sin, and, if they also use contraception, acquitting them of a different grave sin.
What a strange time for the Church! We have some Catholics, Peters inter alia, pretending to defend the Church against one heresy, while asserting a different heresy themselves. “Don’t believe the heresy that contraception is justified by a good intention or by dire circumstances! Instead, believe our heresy, that contraception is only condemned in the case of marital acts!”
In the case of rape, the use of contraception is indirect, and therefore can be moral (given three good fonts). But the other cases of non-marital sex possibly constitute a majority of sexual acts worldwide, and the use of contraception in non-marital intercourse is unfortunately very common. So Peters is saying that contraception is intrinsically evil and always gravely immoral — except most of the time.
Here is my article proving that the Magisterium has in fact clearly and repeatedly condemned the use of contraception, regardless of marital state: Contraception and Heresy — Part 2, the use of contraception outside of marriage. And a second article showing that the Latin text of Humanae Vitae does not restrict its meaning to marital acts only: Contraception and Heresy — Part 3, On the Latin text of Humanae Vitae.
Recent Church documents do emphasize the immorality of contraception in marriage, since all non-marital sexual acts are gravely immoral in themselves. “Historically, however, much of the Christian tradition condemned contraception as a contralife act without distinguishing between its uses by married couples and by others.”  Contraception is not, itself, a sexual act. A person who chooses sex outside of marriage and who chooses to use contraception does two distinct things. A couple who has extra-marital sex has two choices to make: whether to have sex outside of marriage, and whether to use contraception.  Two choices means two possible sins, each grave in this case.
Therefore, “contraception is only contingently related to marital intercourse. For the definition of contraception neither includes nor entails that one who does it engages in sexual intercourse, much less marital intercourse.”  A totalitarian regime (cf. China’s one child policy) might commit the sin of contraception by adding a chemical contraceptive to the water supply. This act is not a type of sexual behavior, and does not presuppose sexual behavior that is specifically marital. And yet it is the grave sin of contraception, condemned by the Magisterium. 
Back to the main point of this article: Dr. Peters has repeatedly asserted the heretical idea that Humanae Vitae, and the Church’s teaching more generally, have restricted the condemnation of contraception to marital acts alone.
In his post About that Humanae Vitae rumor, Peters proposes the same heresy:
“In my opinion the central teaching in Humanae vitae—that contraception between married couples (both terms being correctly understood) is intrinsically evil—is a proposition infallibly taught by the (ordinary universal) magisterium of the Church.”
“What one could imagine being discussed hereabouts is whether the rejection of contraception set forth (I would say, infallibly) in regard to conjugal relations is applicable to non-conjugal relations.” 
Can Dr. Peters evade the charge of proposing material heresy by virtue of his assertion that he is unsure whether the condemnation of contraception is applicable to non-conjugal relations? No, for obstinate doubt in the face of an infallible magisterial teaching is still heresy, even when it is not obstinate denial.
If you say, “Jesus is not the son of God”, then that is a heresy. If instead you say, “maybe Jesus is the son of God, and maybe not”, that is still a heresy.
“Can. 751 Heresy is the obstinate denial or obstinate doubt after the reception of baptism of some truth which is to be believed by divine and Catholic faith….”
Dr. Peters is a member of a growing loose-knit group of Catholics who pretend to be defending the teaching of Humanae Vitae against dissenters — by proposing a radical reinterpretation of Humanae Vitae which limits the condemnation of contraception to marital acts. They are themselves dissenters from the teaching of Humanae Vitae. They dissent by radical revision of the truths taught by Casti Connubii and Humanae Vitae and other documents. Peters himself pretends to have an unswerving devotion to the teaching of Humanae Vitae , but in reality, he is only adhering to his own distorted understanding of that teaching.
True faith adheres to something greater than oneself, something outside oneself. But the faith of so many Catholics today is a fervent devotion to their own limited understanding and misunderstandings, and those of the particular Catholic subculture they have chosen as their own.
The Chiodi argument
At first glance, Peters’ position would seem to differ from that of Fr. Chiodi, who recently asserted that Amoris Laetitia can be applied to contraception . Chiodi would justify some uses of contraception in marriage as well as outside of marriage. Arguing the contrary position, prof. Josef Seifert asserts that “contraception is an intrinsically evil human act that is wrong anywhere and at any time” .
However, whether or not the persons using contraception are married is a circumstance in relation to the act of contraception (as explained above). A sexual act outside of marriage is intrinsically evil, and the fact that the couple are not married to one another is essential to the object of that act. But the choice to use contraception is a contralife choice, not a sexual act. Therefore, the claim that contraception is only intrinsically evil or is only yet condemned by the Church as intrinsically evil, within marriage, causes the morality of contraception to turn on a circumstance, rather than on its object (which is the deliberate thwarting of procreation, that is, an attack on the very beginning of human life).
Thus, Peters is not so different from Chiodi. Both reject the true meaning of Humanae Vitae, understood in the light of all the other teachings of the Magisterium on contraception, on intrinsically evil acts, and on the basic principles of morality. Peters may explicitly condemn Chiodi’s position, but he implicitly adopts a different version of it, one that approves or at least declines to condemn many uses of contraception.
Seifert, on the other hand, holds that “contraception is an intrinsically evil human act that is wrong anywhere and at any time” and that this idea is “solemnly and magisterially laid down in Humanae Vitae, Familiaris Consortio, and Veritatis Splendor”. So Seifert is defending the teaching of the Church, while Peters and Chiodi are attacking it, each by a type of radical reinterpretation.
Reply to Objections
I anticipate some objections to the above argument.
The ad hominem argument: not valid, but very popular nowadays in some Catholic discussion groups, blogs, and Twitter feeds.
Someone might say that those who hold the restricted interpretation of Humanae Vitae (limiting its condemnation of contraception to marital acts) have excellent credentials and good reputations in the Church. My reply is that the Arian heresy is said to have claimed, at its height, about 300 Bishops. And the East-West Schism of 1054 was initiated by the Patriarch of Constantinople (Michael I Cerularius), and was joined by many Eastern Bishops and scholars. So credentials and reputation are no guarantee that the teachings of an individual are free from material heresy.
I am not accusing Dr. Peters of formal heresy. I don’t think he realizes that his position is material heresy. He is well-versed in canon law, and not so well-versed in theology. But then most canon lawyers probably need more training in theology.
Someone might point out that Peters is merely following the thinking of others; it is not a position original to his work. Yes, but that is usually the case with heresy. One person, or a small group, invents the heresy, and then many more persons adhere to its errors.
Some might try to defend this heresy by claiming that the word conjug- (with various endings) in Latin always refers to marriage. But then Casti Connubii stands as a witness against that claim, as it contains three uses of the same Latin word referencing non-marital sexual acts . Humanae Vitae itself uses not only the word conjug- but also this expression: actibus fecundis (“procreative acts”) which obviously includes sexual intercourse outside of marriage, since those acts are also fecund. Quaecumque Sterilizatio uses the Latin term “actuum sexualium” in condemning contraception, and the context is hospitals, which certainly treat unmarried persons as well as married persons; yet no distinction is made in condemning the use of, or cooperation with, contraception by Catholic hospitals.
Some might argue that the Magisterium has no clear teaching condemning contraception outside of marriage. That claim blatantly ignores many teachings, discussed at length here: Contraception and Heresy — Part 2, the use of contraception outside of marriage . See also these authors:
* William May, Contraception, Gateway To The Culture Of Death 
* E. Christian Brugger, Condemnation of Contraception Is a Universal Norm 
* Christopher O. Tollefsen: “contraceptive acts are intrinsically wrong, and not merely always wrong within the marital context.” 
But is Peters’ position heresy, or just a theological error of a lesser type or degree? Well, heresy concerns teachings which must be held with divine and catholic faith, that is to say, infallible teachings. I’m aware of Peters’ claim that some infallible teachings can be denied with an act that is short of heresy. In that schema, an infallible teaching must be proposed as divinely-revealed, in order to require divine and catholic faith (i.e. theological faith).
But Casti Connubii clearly proposes that the teaching condemning contraception is divinely-revealed.
“Small wonder, therefore, if Holy Writ bears witness that the Divine Majesty regards with greatest detestation this horrible crime and at times has punished it with death. As St. Augustine notes, “Intercourse even with one’s legitimate wife is unlawful and wicked where the conception of the offspring is prevented. Onan, the son of Juda, did this and the Lord killed him for it.” ” 
Pope Pius XI teaches that Sacred Scripture is bearing witness to the judgment of the Divine Majesty against contraception, thereby making the teaching divinely-revealed. And in the same passage, the Pontiff teaches, through Saint Augustine, that contraception is gravely immoral “even with one’s legitimate wife” — implying necessarily that it is also wicked outside of marriage.
It is a divinely-revealed truth that contraception is intrinsically evil and always gravely immoral, regardless of marital state. Any Catholic Christian who rejects this teaching, or substantially distorts it, is guilty of at least material heresy.
Sometimes I get questions from my readers, asking me about one Catholic author or another, is that person faithful to the Magisterium? They seek a set of teachers whose writings can be relied upon to be error-free. My answer is that it doesn’t work that way. The infallible teachings of the Magisterium are error free and irreformable; the non-infallible teachings admit a limited possibility of error and reform. But the teachings of individual persons (even the Pope when he is speaking as a private theologian) are fallible and subject to error.
I might disagree with one point that an author makes, and yet agree with another point. I know it’s very common online for Catholics to disparage individuals on the other side of a debate, and to exalt those who are on the same side as them, but that is not truth. I am pointing out a serious error that Dr. Peters has repeatedly asserted, as if it were a legitimate theological position or an infallible magisterial teaching. I do so only to warn the faithful about this particular grave heresy. But he has other posts which are insightful and contribute useful information to the debate on various subjects in Catholicism today. I criticize his posts, but I don’t condemn him as a person.
Please take a look at this list of my books and booklets, and see if any topic interests you.
 Misunderstanding the (alleged) ‘Congo contraception’ case;
 Contraception and Heresy — Part 2, the use of contraception outside of marriage;
 Contraception and Heresy — Part 3, On the Latin text of Humanae Vitae;
 Why Is Contraception Always Wrong?
 Every Marital Act Ought To Be Open To New Life;
 ibid.; cf. Familiaris Consortio, n. 30; Compendium, n. 234.
 About that Humanae Vitae rumor;
 What I Owe to Blessed Pope Paul VI;
 Re-reading Humanae Vitae (1968) in light of Amoris Laetitia (2016);
 Professor Seifert Comments on Fr. Chiodi’s “Re-Reading of Humanae Vitae”;
 William May, Contraception, Gateway To The Culture Of Death;
 E. Christian Brugger, Condemnation of Contraception Is a Universal Norm;
 Christopher O. Tollefsen: “contraceptive acts are intrinsically wrong, and not merely always wrong within the marital context.”
 Casti Connubii 55.