You may recall from my first post on this topic that "BV" officially stumped me regarding whether or not material sin results in a moral evil. My initial thought was to say yes to this question, based on the objective "rightness" or "wrongness" of every action. After all, it is true that certain actions are sinful regardless of our knowledge of their sinfulness. But, "BV" pointed out some information in the very sources I used to support my position that seemed to say that moral evil need not result from such actions, despite their objectively sinful nature (see his comments to my original post). Today I was finally able to talk to a professor about this, and I think I understand things a little better, although I admit that my thoughts still need more definition.
Basically, from what I gather, I was wrong. A moral evil results when we willfully do something that we know to be wrong. If our conscience informs us that an action is wrong and we do it anyway, we sin. Note that moral evil results from a sin knowingly and willfully committed.
Now, in the case of someone who performs an action without knowledge of it's sinfulness--although objectively speaking, or at the level of the abstract, his action is morally wrong--the result of his action will not be a moral evil because of the absense of knowledge that was present. Conversely, if, objectively speaking, an action is morally permissible but a person's conscience is so formed that he is certain the action is morally wrong, then if he performs this act he sins, and moral evil results.
Keep in mind the definition of moral evil: the willful performance of an action to which the conscience objects. Even though in all reality his action was permissible, we must not forget that malice is the motive behind any action performed in the face of knowledge, however faulty, that an action is wrong. Such a response is always sinful and results in moral evil.
I'm pretty sure that this is the gist of what my professor told me. If I discover later that corrections need to be made, I will let you know. Perhaps I'll email him and see if he agrees with what I have written here. What do you think about this? Let me know.