It is at best simplistic to presume that because someone hears either Father say, or a religious educator say, that the Church teaches that ABC is morally wrong, that they therefore have sufficient knowledge, and assuming that the other requirements for a mortal sin - serious matter (it is) and sufficient will (see below) ar present, that they have committed a mortal sin.
All of that , and the posts above which posit it, are based on an unspoken assumption, one that is easy to make but hard to identify, and that is that most of the posters in these fora accept the teaching authority of the Church, and specifically that aspect of the teaching authority which defines for us what is a serious matter.
Back up a few steps and look at that assumption; you cannot leap from that to a simple statement that most Cahtolics using ABC operate from that basis.
Many Catholics have little or no connection with the Church; they were baptized (and it is truly amazing how many people who were baptized Catholics and never went any further identify themselves as Catholics), and possibly received First Communion, and never made it to any further official instructions; or their instructions never made it much farther than through grade school.
Anyone familiar with catechesis over the last 40 to 45 years (that is, going back to about 1970 or thereabouts) would know that catechesis pretty much went into the toilet, and then someone flushed. Adults - even those who attend regularly - know little about the Sacraments, or Church law, or Church authority. Precious little was taught about morality unless it had to do with the context of the social Gospel. Sexual Morality, if it was taught in high school classes (and realize there is a significant drop off between 8th grade and freshman in CCD equivalent classes) at all, was perhaps one class in 4 years, and that may have lasted 1/2 hour.
Meanwhile, each and every one of them has been living in the world, a world of promiscuity, abortion, divorce, and each and every other impact to their moral behavior, and many, if not most, simply see the Church as one more voice. Not the only voice, not the voice of Truth, but one more voice among the cacophony of all the others, and often largely irrelevant. Most of them have little if any contact with the Church concerning the issue of ABC until not just the choice for or against ABC, but the very foundations of making that choice have been well and truly settled in. Then the Church comes along and says, for a half an hour or an hour in a life that is 131,400 hours, or 157,680 hours, or 183,690 hours, or 219,000 hours old (thats 15, 18,21 or 25 years old) that ABC is wrong. And it is saying it to someone who already has a whole serious of decisons made, such as “who do I listen to for advice”, and “what does the Church know about xyz”.
Coupled with this is the dissent that has gone for 40+ years since the issue of Humanae Vitae; not only have they and their peers not heard of it let alone read it, but the generation before them most likely has never read it and can’t remember it’s name. So you have those coming up for marriage, coming from a family that accepted ABC and rejected (or simply never learned) what the Church taught.
Persuming that they have a correctly formed conscience is bordering on ludicrous, and unless they have a correctly formed conscience, it is, to put it politely, somewhat hard to make the case that they have committed a sin. That it is objectively sinful I think we can all agree. That they even have a slight hint of that is significantly debatable.
