You need to grasp the difference between sin and personal culpability.  

"Deep within his conscience man discovers a law which he has not laid upon himself but which he must obey."

Because a person "thinks" or "feels" something is not sinful does not make it so. 

" ... 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." In such cases, the person is culpable for the evil he commits." 

"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."