CathInfo.com

One might notice, however, that TIA was not primarily answering the question alluded to in the thread title, whether it's a mortal sin TO attend the New Mass, but, rather, they were answering the question whether it's a mortal sin NOT to attend the NOM if it's all that's available to you on a Sunday, i.e. whether the Sunday obligation requires attending the NOM.  They give a clear opinion on the matter that NO, it is not because the New Mass is "an activity that has the flavor of heresy and for this reason is highly offensive to Our Lord."

So ... "he must avoid going to this Mass so as not to fall into the sin of promoting an activity that has the flavor of heresy and for this reason is highly offensive to Our Lord. Therefore, he does not commit mortal sin by missing the Novus Ordo Mass."

It says here clearly that if someone KNOWS about the problems with the NOM, then "he must avoid going to this Mass".  This is the subjective vs. objective distinction, the stipulation that if the person knows and is aware of the problem with it.

So the Mass itself, they say, is "an activity that is highly offensive to Our Lord" (objectively very bad) and so one must avoid it if one knows it to be bad (subjective).

Both Mgr Williamson and Fr Pfeiffer have said that it is ok to attend the Novus Ordo. Therefore it is not a sin to attend the New Mass. 

There are many distinctions in play, Stubborn.  So, for instance, objective vs. subjective mortal sin.  That is a huge distinction.  One could be certain that something is objectively a mortal sin but be unsure of the state of that person's conscience and whether that person was actually committing mortal sin before God.

Subjective or objective mortal sin does not nullify the sin, it might only change the degree of personal culpability, which only God knows to what degree.

Hell is full of souls who died in both, objective and subjective mortal sin.

I do not and never have understood why is it that trad priests and even bishops, who dislike, hate, even abhor and would not be caught dead saying the new mass themselves do not condemn with one clear voice, that going to the new mass is a mortal sin. Perhaps because, like EENS, they don't believe it themselves, which is crazy.

"all you need to do is consider what has happened to the faith of those who have been attending the new mass through the years".