A grave sin is always a grave sin, but the person’s cupability may mitigate his personal responsibility. It is the duty of all Catholics to be informed about Church teaching no matter how well they were catechized as children/young adults. There’s plenty of good information available for all, so really, unless a person is illiterate or simply unable to grasp information, no one really has a good excuse for violating Church teachings.
I’ve debated with fellow Catholics on this forum who refuse to be corrected even when presented with proper Church teaching simply because they don’t want to accept it, but want to go with what they want to think. Why they do that is between them and God, so I cannot judge their motives, but I do fear for them when they have to stand before God if their motives were to willfully defy what they knew the Church taught.
In the end, only God can decide between the heart, mind, soul, and spirit for he knows us inside and out, and better than we know ourselves. People can so easily deceive themselves and make excuses that going against what they know to be true only adds to their cupability.