Recently discovered in the desk of a (badly missed) colleague: the 1962 pamphlet How to Settle Your Conscience, by Hugh J. O'Connell, C.SS.R. It contains some illuminating passages:

In danger of harm to our own or another's soul or body, we must ordinarily take the safe side. For example, John has a serious doubt as to whether attendance at a certain show will cause him to fall into grave sins of impurity. It would be wrong for him to attend, since he would be placing himself, without sufficient reason, in danger of grave sin.So too, if a girl suspects that a certain manner of dress or acting will cause serious temptation to her boy friend, she must take reasonable care not to cause such temptation.It is, nevertheless, allowed for serious reasons to permit danger of scandal. Thus, to safeguard the innocent, a man would be allowed to make known the immorality of a certain night club, even though as a result some persons might go there who would otherwise not have known about it. In such cases, one must balance carefully what best promotes the common good.(...)But what should we do when in doubt about the morality of an action?... What if we can't find out the truth at the moment?... Mary is at the Communion rail. She suddenly remembers that she took a piece of candy. She knows it was about three hours ago, but she is not sure of the exact time. May she receive Communion? The answer is yes, because she must make the decision at once, and, until it becomes certain that she ate the candy less than three hours ago, she is free to go to Communion....Susan is a married woman. She gets a doubt whether a certain manner of lovemaking is right or wrong. May she cooperate? At the moment, since she is not sure the action is wrong, she would not be obliged to refuse her husband. However, she would be obliged to discover as soon as she reasonably could whether the action is objectively right or wrong.(...)If a person is ignorant without his fault, he is not guilty of sin in doing something that he thinks is right, even though it is really wrong. An Eskimo, for instance, has been brought up to believe that old people should be abandoned on the ice to die. He has no suspicion that this is wrong, so he is not guilty of sin in following his conscience, even though it is erroneous.But it is not only savages who are ignorant of the moral law. A tremendous number of civilized people have wrong ideas about morality because of false education, or no education at all, about moral matters.