Well, Bishop Williamson only thinks he shouldn't have said this publicly. This sounds to be a hair's breadth removed from the Bergoglian doctrine where you can denounce divorce / remarriage as wrong or sinful objectively and in the "external" forum, while "discerning" a different outcome in the "internal" forum by working it out with your Confessor.
This is subjectivist moral relativism
but it's either objectively right or wrong to assist at the NOM.
If I don't know it's forbidden to eat meat on Good Friday, I wouldn't sin by eating meat. But if I ask a Priest or Bishop in public whether it's permitted to have a steak dinner on Good Friday, the response can't be "Well, if you think it's OK to eat meat on Good Friday, then make sure to get a good quality steak." Said Bishop/Priest must inform the ignorant or otherwise malformed conscsience.
Ladislaus, I think there is a problem with your moral theology. Now I am not a theologian either, so I need to be careful here. I would invite any priest who is on the forum to clarify the theology for us.
Let us consider a few examples to shed some light on the issue.
Thou shalt not commit adultery. Here we have a Commandment of God. It permits of no exception. Similarly, divorce - "what God has joined together, let no man put asunder". You denounce it from the pulpit, and your advice in the confessional to any given soul, no matter what his circuмstances, even if he is the King of England, is identical, whatever Pope Francis may have said to the contrary.
Thou shalt not steal. Here, another Commandment of God. Yet it does admit of an exception. You denounce it from the pulpit, but your advice in the confessional to a poor soul who confesses that he stole an apple from a tray at the markets because he had no means and had not eaten for days would be "be of good heart, my son, there is not fault there", clearly against the letter of the Law, yet in this case not sinful, for the God who gave us life wants us to have the necessary goods to sustain that life.
Thou shalt not attend the
Novus Ordo Missae. Why? This is not a commandment of God, yet nonetheless it may be true. Why? If we take the NOM as it exists today, in practice, the very first reason I would give is that the priest is probably a doubtful priest and therefore it very well may not be a Mass at all. Now, straight away, the reason for giving that advice "thou shalt not attend the New Mass", admits of an exception (presuming the absence of any other reasons to abstain from attending): "however, if it can be determined that the priest was certainly validly ordained, then one may attend". The statement "thou shalt not attend" is not necessarily absolutely true without qualification. Yet, if there are qualifications to be made, if there are exceptions, it may very well be prudent to make those known to individuals in private rather than in public which may run the risk of causing scandal.
Thou shalt not attend the NOM. Why? In the early days of its promulgation, when we could presume that most of the priests offering it were true priests (and before the clown 'masses' and spontaneous 'innovations'), the only reasons given by Archbishop Lefebvre that I am aware of, were 1. danger to the Faith, and 2. risk of scandal. The question then arises, is there always danger to the Faith of a given individual, and is there always risk of scandal to one's neighbour? If not, it may well be that one can attend. There could conceivably be a parallel here with the example above of "stealing". In spite of being against the letter of the Law (or in this case, the general advice not to attend the New Mass), it may well be the best way for a given individual, in certain circuмstances (spiritual starvation) to keep himself alive spiritually, as beautifully explained by Archbishop Lefebvre in the spiritual conference posted by Sean above, and more recently repeated by Bishop Williamson.
Thou shalt not go to the beach (that is, popular swimming spots). Here we have another prudential, moral judgement (not a dogmatic one). Ninety percent of the women folk reading this won't have a clue what I'm talking about, while ninety percent (if not 100%) of the men will know exactly what I mean. As one priest told us on a men's retreat "if you can go to the beach and not have a problem, see a doctor, there is something wrong with you". Again, we ask the question, why? It is not a Commandment of God, yet it is true to say. The Commandments of God come under the numbers 6 and 9, and generally, it is a proximate occasion of sin against these Commandments for the men, and the women, with their dress, are responsible (even though they don't usually understand...). However, not everyone is the same. It may be that for a given individual this is not a proximate occasion of sin, and in the confessional he may explain to the priest that he has a festering ulcer that his doctor advised him to bathe in the sea water, and he may be given permission to do so.
There is no hair's breadth of a difference between the advice of Archbishop Lefebvre and Bishop Williamson on the New Mass, and the "situation ethics" or "moral relativism" or "subjectivism" of the modernists. They are light years apart, one is Catholic, the other undermines Catholic Truth and Catholic morals.