I’m not sure what you’re looking for, AC. If after considering *all *the facts, a person knows doing a certain action would be wrong, then they should not do it, out of obedience to conscience.
Grant for a moment that someone is Catholic and knows that the Church teaches X to be gravely wrong in all cases. Furthermore, that someone knows that the Church speaks with real authority, etc. Then they can’t just claim their conscience says it is okay to do X because considering all the facts includes considering what the Church says. And if they know enough about the Church to understand its authority, then they know enough to know that X is wrong. They’d have to bury their head in the sand to say that X is okay, and burying your head in the sand does not excuse.
Of course, there are sadly too many Catholics out there who are very poorly taught and don’t realize it. So they may not really know about the Church and her authority or her teachings. It is their duty to continue to learn about the faith, however, so they may not be excused if they are derelict in that duty. But in some cases they are (excused).