Do you have a reference for that? Either biblical or from official church teaching via the CCC. The bible tells us that some ignorance mitigates the gravity of the sin but I am unfamiliar where our faith teaches that it removes ALL guilt.
Yeah, no problem. If you check my post that you quoted there is a selection from the Summa Theologiae–which is online if you would like the reference-- St. Thomas Aquinas, is where much of moral theology from the CCC comes from, either directly (as in this case) or indirectly. I
t is called invincible ignorance.
If you don’t buy that-- the CCC says
1790 A human being must always obey the certain judgment of his conscience. If he were deliberately to act against it, he would condemn himself. Yet it can happen that moral conscience remains in ignorance and makes erroneous judgments about acts to be performed or already committed.
1791 This ignorance
can often be imputed to personal responsibility. This is the case when a man "takes little trouble to find out what is true and good, or when conscience is by degrees almost blinded through the habit of committing sin."59 In such cases, the person is culpable for the evil he commits.
1792 Ignorance of Christ and his Gospel, bad example given by others, enslavement to one’s passions, assertion of a mistaken notion of autonomy of conscience, rejection of the Church’s authority and her teaching, lack of conversion and of charity: these can be at the source of errors of judgment in moral conduct.
1793* If - on the contrary - the ignorance is invincible, or the moral subject is not responsible for his erroneous judgment, the evil committed by the person
cannot be imputed to him. It remains no less an evil, a privation, a disorder. One must therefore work to correct the errors of moral conscience.*
1860
Unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest.
These concepts can also be seen in the following Scriptures:
John 15:22 If I had not come and spoken unto them, they had not had sin: but now they have no cloak for their sin.
John 9
35 Jesus heard that they had cast him out; and when he had found him, he said unto him, Dost thou believe on the Son of God?
36 He answered and said, Who is he, Lord, that I might believe on him?
37 And Jesus said unto him, Thou hast both seen him, and it is he that talketh with thee.
38 And he said, Lord, I believe. And he worshipped him.
39 And Jesus said, For judgment I am come into this world, that they which see not might see; and that they which see might be made blind.
40 And some of the Pharisees which were with him heard these words, and said unto him, Are we blind also?
41 Jesus said unto them, If ye were blind, ye should have no sin: but now ye say, We see; therefore your sin remaineth.
Romans 2
7 To them who by patient continuance in well doing seek for glory and honour and immortality, eternal life:
8 But unto them that are contentious, and do not obey the truth, but obey unrighteousness, indignation and wrath,
9 Tribulation and anguish, upon every soul of man that doeth evil, of the Jew first, and also of the Gentile;
10 But glory, honour, and peace, to every man that worketh good, to the Jew first, and also to the Gentile:
11 For there is no respect of persons with God.
12 For as many as have sinned without law shall also perish without law: and as many as have sinned in the law shall be judged by the law;
13 (For not the hearers of the law are just before God, but the doers of the law shall be justified.
14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:
15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another

16 In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.
Ergo: sometimes it is your own fault in which cases you are held responsible; because the natural law could be found out by the person if they considered the matter (as I tried to make clear before that ‘they could have known’) they would have known that it was evil and indeed in these matters it is that person’s duty to find out what is to be done.
Sometimes, however, it is not your fault that you do not know what you are doing is wrong and you are not culpable. When there is a certain amount of separation between natural law principles and the particular sin itself then the person might remit his guilt. Fornication is opposed to natural law, people choose to fornicate even though they know -or-could-know- that it is wrong. People often contracept not knowing that it is wrong- I am sure that there are many non-Catholics and non-Christians in this predicament. This does not make contracepting to be okay, no. Right and wrong are not relative. But culpability for doing what is wrong CAN be relative to the person who is committing the act. This can diminish the sin such that it is a venial sin (which while it disposes one to mortal sin does not separate him from the love/grace of God) or it so that it is not a sin.
Once a person has been told, or had the opportunity to have been told, that what he is doing is wrong then his ignorance is no longer invincible and he is culpable for his sin in varying degrees.