Is it possible for a married Catholic to be invincibly ignorant that adultery is gravely wrong?

Is it possible for a married Catholic to be invincibly ignorant that adultery is gravely wrong?

The short answer is no.

Seeing that the vast majority of Catholic dioceses, if not all, have a mandatory marriage preparation to take before getting married, it is highly doubtful that an individual Catholic would would be invincibility ignorant that adultery would be gravely sinful.

The knowledge about such, as Geremia points out is one thing, but the Church has taken steps to ensure that Catholic marriages are valid from the beginning and remain so until the death of one of the spouses!

The answer is very much context dependent. In principle it is logically possible for a Catholic to be invincibly ignorant about any moral fact. But it is incredibly difficult for a Catholic person (or any other person) to be ignorant of the fact that adultery is wrong.

Given the visibility with which the Catholic faith --- and indeed, Christianity and even the majority of secular American culture (and all other cultures) --- teaches about the importance of fidelity in marriage, such a Catholic would have to be extremely badly catechized, and extremely mentally deficient in absorbing the moral law regarding marriage even though such a Catholic is surrounded by it on all sides. So, possible? Sure. Likely? Not really.

On the other hand, perhaps your question is emphasizing the "gravely" in "gravely wrong". It seems more likely to me that an extremely badly catechized Catholic with an extremely deficient reasoning ability and an extremely badly trained moral compass might come to the conclusion that adultery is wrong, but maybe not "gravely" wrong. After all, people are very good at self-deception when it suits them, and I can imagine a Catholic --- or indeed any one person --- downplaying the gravity of any sin (including adultery) if it is something they want to do. Add to this that an extremely badly catechized Catholic might not even be aware of the distinction between "wrong" and "gravely wrong", and might be (miss-)guided in confusing their feelings for their moral reasoning --- as seems to be the case for many modern Americans ---, and the likelihood goes up for such a Catholic to conceivably conclude that even though adultery is wrong, maybe it is not that wrong because it causes them "feelings" of "love".

All of this to say that questions of invincible ignorance can be quite subjective, and circumstantial. It does really depend on the person and the kind of education (formal and cultural) to which that person has access.

Here is how I might be wrong. The argument can be made that since the prominence for the proposition that adultery is very wrong is so high, a Catholic (or any other person) would have the moral obligation to investigate the truth or falsehood of it --- and hence could not be invisibly ignorant (but merely ignorant) if he did not.

Despite this counterargument, I still think that an extremely badly catechized Catholic, with extremely deficient mental and moral abilities, and with an extremely bad access to the true moral law --- perhaps all the friends and teaching authorities (even those who should know better) of such a Catholic teach that "true love trumps all else" --- could possibly come to be invincibly ignorant about adultery being a "grave wrong". Of course, the more visible and prominent that the teaching against adultery is, the less likely it is for such a Catholic to exist.

A (validly) married Catholic cannot be invincibly ignorant of the sinfulness of adultery because marriage consent, "the efficient cause of matrimony" (Summa Theologica suppl. q. 45 a. 1), is consent to "the union of persons directed to one purpose" (ibid. ad 2; cf. q. 44 a. 1), or as Canon Law expresses it:

1917 CIC 1081 (1983 CIC 1057)

2. Matrimonial consent is an act of the will by which each party gives and accepts perpetual and exclusive rights to the body, for those actions that are of themselves suitable for the generation of children.

Regarding ignorance:

1917 CIC 1082 (1983 CIC 1096)

1. In order that matrimonial consent be considered [valid], it is necessary that the contractants at least not be ignorant that marriage is a permanent society between a man and woman for the procreation of children.

2. This ignorance is not presumed after puberty.

cf. Summa Theologica II-II q. 154 a. 2 "Whether simple fornication is a mortal sin?"