Behind your answer, I’m seeing that your underlying basis for your thought no one can sin unless they believe it’s a sin is: No action a person can take is sinful unless the person taking that action thinks it’s sinful. Do you have any references you can provide from the Bible or the Catechism that substanitates this line of thinking?
Using that line of thinking there is no objective right or wrong. A person who has an abortion isn’t sinning when they kill their unborn child since that don’t think it’s wrong. A person who rapes hasn’t committed a sin since he thought it was his right. A child molester hasn’t sinned because he thinks it’s alright.
I see that in the Catechism # 1860 says that “Unintentional ignorance can diminish or remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, written in the conscience of every man.” At most, an otherwise moral sin could be lowered from grave/mortal status yet the sin remains it remains a sin. The Catechism gives Sin an objective definition in # 1849: “It has been defined as “an utterance, a deed or a desire contrary to the eternal law.”” In addition, Jesus did say we will also be held guilty for sins not physically committed, but also sinful thoughts when He said as recorded in Matthew 5:28, “But I say to you that everyone who looks at a woman with lust has already committed adultery with her in his heart.”