Well, thanks for being sorry, but I am sorry too, that your opinion that to break the law of abstinence is a venial sin has been anathematised and condemned by the Pope. …“the law of abstinence embodies a serious obligation whose transgression, objectively considered, ordinarily involves a
mortal sin. The unanimous verdict of theologians, the constant practice of the faithful, and the mind of the Church place this point beyond cavil. They who would fain minimize the character of this obligation so as to relegate all transgressions, save such as originate in contempt, to the category of venial sin are
anathematized by Alexander VII [Cf. Prop. 23, ap. Bucceroni, Enchiridion Morale, 145 (Rome, 1905)].”
Further…“In fine, the Trullan synod (can. 58, ap. Hefele, “History of the Councils of the Church”, V, 231, Edinburgh, 1896) inflicts deposition on clerics and
excommunication on laymen who violate this law. Furthermore, theologians claim that **a grievous sin is committed as often as flesh meat is consumed in any quantity on abstinence days **(Sporer, Theologia Moralis super Decalogum, I, De observ. jejunii, # 2, assert. II),
because the law is negative, and binds semper et pro semper. In other words, the prohibition of the Church in this matter is absolute.”
newadvent.org/cathen/01067a.htm