Well, any one person is just that–one person. One limited human person, of limited human intelligence, limited human ability, limited time allotment on earth, limited language, limited culture, limited physical and mental capacity, etc.
The Catholic Church is the body of Christ. It is made up of all baptized Christians, living and dead. It comprises limited human beings from over a 2000 year period of history, of varying degrees of physical and mental capacity, of all known human languages, cultures, etc.
AND IN ADDITION. . .
The Church has within it to guide it the Holy Spirit, who proceeds from the Father and the Son and who was sent to reveal all necessary knowledge.
While I might be Stephen Hawking (but I’m not), or Albert Einstein, or Ken Jennings in “brain capacity”. . .
While I might be Mother Teresa, St. Theresa, St. Peter or St. Paul in “spiritual capacity”. . .
While I might be AH-nold or Charles Atlas or Babe Didriksen Zaharis in “physical capacity”. . .
Any, or all, of the above, while “gifted” in a given area, are, even in the “spiritual” capacity, still limited human beings. (Ask St. Peter especially).
So, your example of an individual with a “well formed conscience”–is limited by his own personal finite human capacity. This person is likely to be Joe or Jane Average far more than the above examples, so even less likely to have a “gift” in one way or another, especially a spiritual gift. . .
The Church, OTOH, has 2000 years of Holy Spirit guidance, Scripture, Tradition, and teaching.
What IT teaches, despite what might be politically correct or just seem “not a sin” to John, Jane, or anyone in between, is not just “one person’s opinion”.
You’re not just talking about things like communion in the hand or whether to sing Haugen or Palestrina. . . one can in the U.S. receive communion in the hand OR on the tongue, and one can choose either of the above composers legitimately depending on individual taste.
The examples that you give are quite specific moral teachings that the Church has held since the beginning, and always will. Unlike women wearing hats in church, to give an example of an excellent idea that lasted for centuries and could still be done quite legitimately today, but equally legitimately need NOT be done, adultery, murder, and contraception are moral wrongs that can NEVER be engaged in.
Any person whose “well informed conscience” informed him or her that those were NOT sins, but if the Church says they are, will go with the church, is doing the right thing for the right reason but is still begging the question. Consciences do not “spring into being” overnight and at any time, not just this era, one must take particular care not just to “go along with the teaching” but to ACCEPT it wholeheartedly.
People learn things every day, and in many cases, in order to learn something new, they have to reject something “old”. A child looks at a short, fat glass and a tall, thin glass and will tell you that there is more water in one because it is taller, even if there is exactly the same amount in both glasses, because to her it LOOKS as though there should be more water. As she grows older she learns that looks are deceptive. Should she have held to her “conscience” because she really THOUGHT she was RIGHT? Should she have held to her opinion rather than the teachers because teachers are not cool, because of the teachers’ sex, or race, or religious beliefs?
Mr. X and Ms. Y may really THINK that adultery is OK. That does not make them correct. They need to LEARN that adultery is NOT OK, not to engage in the relativistic babble that, well adultery seems OK to THEM, and probably society, or at least the intelligensia thought so TOO, at one time, or do NOW, so really, isn’t it more IMPORTANT to stick to one’s moral principles, however flawed, rather than to “kowtow” and actually reject those principles, in favor of some ABSOLUTE?
Honestly, more people have fallen victim to the sin of pride and hubris in considering that what they “believe” somehow trumps something that is “taught” to them. . .