In no way am I saying adultery is good or not grave matter; but do you understand what mortal sin is and what it required for a person to be guilty and/or culpable of that sin?
If a person out of no fault of his own is not exposed to this truth he is not guilty of the sin. That is straight from the CCC.
We are required, to follow our conscience. With that big and important a responsibility comes the requirement to form that conscience, yes. But on the same note, if a person is never exposed to the fullness of truth, how can he be held to that truth? This is in fact Catholic truth and doctrine. Read and study the whole section on the conscience, there is as much requirement to follow our conscience as there is to properly form the conscience.
Liberal thinkers tend to concentrate on the “following” and for get about formation; conservative thinkers think of the requirement of “proper formation” more than following the conscience. We must remember balance.
vatican.va/archive/ccc_css/archive/catechism/p3s1c1a6.htm
A person, who does not know of a grave sin, honestly has no way of knowing, is not guilty of the sin; this is fact. In the same way if a person is not free to choose the sin and is forced to do it, there is no sin.