Nowadays, by far the most unpopular doctrine of the Church is that all volunary genital activity muct be of the procreative sort, even if one isn't aiming at procreation and even if one or both parties are sterile through no doing of their own. In other words, sexual activity must culminate in vaginal intercourse with male ejaculation in the vagina and without any steps taken to interrupt or block the generative process (if any is possible) either before, during, or after the act. If that doctrine is false, then the Church's condemnation of homogenital sex and contraception, which is consistent for as far back as we have records, is as mistaken as the majority of people now believe it to be. And if that is so, then there is no particular reason to accept the Magisterium's distinctive claims for itself at all. One claim sometimes made for taking that stance is that the Church once taught that all sexual activity is somehow morally defective; even marital intercourse meeting the above criterion is at least "venially" sinful because, in our fallen state, it cannot but occasion the sin of lust. And since we "now know" that to be false, the Church's having taught such a thing is good reason not to accept the above criterion either.
Well, here is my reply made at Pontifications to one good Catholic who, while not going quite that far, does see a problem here:
Among Catholics at any rate, I have found the kind of problem you’re raising is probably the most common for the assent of faith, as distinct from this-or-that particular point of faith. But I have also found that the best way to deal with it is to attend carefully to what is actually being proposed on this-or-that particular point. When one does, it almost always becomes clear whether or not one is faced with (a) a matter pertaining to the deposit of faith and (b) a matter of that kind on which the Church has taught definitively.
Consider your favorite example, which I don’t think is the most difficult. We must first ask whether the claim that “sex is always sinful” is a claim about what sex intrinsically and necessarily is in itself as distinct from what it is in the condition of fallenness. If Augustine and Gregory had been claiming the former, they would have been Manichaean heretics, not Christians, who believe that a perfectly good God created the material world and all he created is necessarily good. But none of the Latin Fathers were Manichaeans. Augustine did have a Manichaean phase which some claim carried over into his view of human psychology, and there is some truth to that criticism. But what he really claimed was that, given the effects of original sin, all our faculties are corrupted to some extent—the spiritual no less, and perhaps even more, than the animal. Therefore, even for the person being transformed in Christ and thus sanctified, venial sin is inevitable in every sphere of life. That holds especially of sexuality inasmuch as that faculty can so easily override reason and it is the medium by which human life, and therefore original sin, are transmitted. But the question to what extent the residual effects of the Fall make sin inevitable is not one pertaining to the deposit of faith. One can say that some-or-other venial sin is inevitable in the exercise of any given faculty without thereby claiming that each-and-every exercise of that faculty is venially sinful. The former is a logical consequence of the irreformable teaching of the Church; the latter is and must remain a matter of opinion. Accordingly, the latter forms no part of the subject matter on which the Church as such either can or proposes to teach definitively. One may disagree with Augustine and Gregory by claiming that a married couple do not necessarily sin venially every time they have sex. Especially in our overeroticized and contraceptive culture, they can and do sin venially sometimes. But sometimes they make love not lust.
There are many other examples of teachings that were once common opinions of churchmen but have since been abandoned. I treated some of them in my article series on Development and Negation. If this is the sort of issue that deeply concerns you, there is no alternative to the kind of analysis I’ve been doing—case by case by case.
Readers might find the entire thread of interest.