Regardless , it’s not worth being a mortal sin.
I don’t eat meat on Fridays of Lent because I’m Catholic and it’s a way for me to remind myself of my Catholicism. I don’t avoid meat out of fear of sin. In fact, I never knew it was a sin until reading it on CAF awhile back, and I’m 50 years old. I’ve mentioned the Meat as Mortal Sin ito other Catholics and they ask, “You’re kidding, right?”
The relationship between ecclesial law and moral law is like the relationship between Mosaic law and moral law.
Mosaic law had multiple aspects, such as ceremonies and custom, all of which had a particular purpose in the context of the era before Christ. Many of the customs were in order to separate the people of Israel from others, and the ceremonies and sacrifices were the shadows of a promise by God to cleans us of our sins.
However, with the coming of Christ, especially his Sacrifice and his opening of the new convent with gentiles, these ceremonies and customs were fulfilled and became without purpose in the context of the last days which we live in now. And so, we have been unbound in following them: in fact, to follow, say, the rituals of the Temple Sacrifices, would be acting as if Christ Sacrifice didn’t cleans us of our sins!
However, just because a new context arose, doesn’t mean that the people of God wouldn’t need guidance in particular customs and how particularly to celebrate the Liturgy in order to promote unity and order and harmony among the members of Christ. He foresaw this need, and so he imputed to the Apostles the authority to “bind and loose” the Church through the guidance of the Spirit and his gifts of wisdom and prudence in a way similar to how He himself guided Moses to bind and loose Israel by the old testament law. This is where the Church is coming from when the bishops make certain days holy days of obligation, or Wednesdays and Fridays fasting days, or change the details of the Liturgy, etc.
These decisions are in a sense customary and arbitrary, as result of prudence and discernment. They do not derive from the moral law necessarily, but they can bound how the Church approaches certain moral issues practically, like how, for the longest time, only bishops were allowed to absolve the sin of abortion, just like how Moses bound how Israel was to deal with adultery. The moral law and its principles are not changeable, and so adultery and abortion are always evil in every circumstance, but how we deal with it from a practical standpoint in different situations is.
This is why the Church cannot just make anything prohibited by the natural law just. The bishop is simply exercising this ability to bind and loose his diocese regarding the specifications of the general principles expressed in both the moral and Divine laws.
Christ says we should fast, and the bishops say we should fast on Wednesdays and Fridays for a multitude of practical reasons, such as the understanding that fasting at the same times every week help develop the habit, promote unity in the Church (since everyone is fasting together), giving Christians more ability to lean on each other for help when the temptation to break fast arises, to separate the people of God from non-Christians and Jews (Wednesdays and Fridays we’re chosen, according to the Didache, in order to separate Christians from religious Jews, who fasted on Tuesday and Thursdays), and so on. Most of these intentions in the binding of this law can be fulfilled in another way, but they are all fulfilled in this way as well, and if the situation changes that it would be more prudent to loosen this tradition of men, than the Church should and could do so.
These bindings are mortal sins in the sense that a Jews eating pork is a mortal sin: the act isn’t inherently wrong like murder or adultery are, but they are wrong because Moses or your bishop binded it to be so, for whatever practical reason(s).
We just must always remember that we shouldn’t allow these traditions of men, although good in themselves, conflict with the traditions of God.
Does that help make more sense of these seemingly arbitrary customs?
Christi pax.