Let's be blunt.
The bishop followed the teaching of Aquinas, the priest did not.
Recall that simple fornication is a sin in significant part because it is opposed to the good of the child that might be born:
Summa Theologiae II-II, 154, 2
Reply to Objection 4. Simple fornication is contrary to the love of our neighbor, because it is opposed to the good of the child to be born, as we have shown, since it is an act of generation accomplished in a manner disadvantageous to the future childBut, of course, comparing fornication to murder in such a way as to make fornication a sin equal to or greater than murder is explicitly rejected by Thomas himself:
Summa Theologiae II-II, 154, 3
Whether fornication is the most grievous of sins?
On the contrary, Gregory says (Moral. xxxiii, 12) that the sins of the flesh are less grievous than spiritual sins.This was a commonplace among medieval Christians. This is why Dante places adulterers and fornicators, and all those bound by natural lusts, in the second circle of hell while placing murderers much deeper, in the seventh circle of hell.
I answer that, The gravity of a sin may be measured in two ways, first with regard to the sin in itself, secondly with regard to some accident. The gravity of a sin is measured with regard to the sin itself, by reason of its species, which is determined according to the good to which that sin is opposed. Now fornication is contrary to the good of the child to be born. Wherefore it is a graver sin, as to its species, than those sins which are contrary to external goods, such as theft and the like; while it is less grievous than those which are directly against God, and sins that are injurious to the life of one already born, such as murder. (emphasis added)
If traditionalists had a clue, they would applaud the bishop for following traditional nuances and for correcting a priest who was trying to twist Catholic teaching to suit his own agenda. But, once again, traditionalists seem not to have read done the reading.
So, once again, we have to wonder what is so "traditional" about so many of our celebrated "traditionalists"?