The thread is not about if man sins mortally, but why. The Church teaching is that mankind knowingly and willingly
rejects God, it is called mortal sin. St. Pope John Paul II wrote of those that mortally sinned: breaking the original covenant:
Committed after original sin, personal sins are conditioned by the state of inherited inclination to evil (“the spark or incitement of evil desire”), in a certain sense already at the very point of departure. However, this situation of inherited weakness does not cancel human freedom. Every actual (personal) sin is a real abuse of freedom, contrary to the will of God. …
Speaking about Adam’s sin, St. Paul describes it as “disobedience” (cf. Rom 5:19). The same is valid for every actual sin committed. Man sins by transgressing God’s commandment, therefore he is “disobedient” to God as supreme lawgiver. In the light of revelation this disobedience is at the same time a breaking of the covenant with God. God, as we know him from revelation, is the God of the covenant. Precisely as God of the covenant, he is lawgiver. He inserts his law in the context of the covenant with man, making it a fundamental condition of the covenant itself.
Thus it was in that original covenant, which, as we read in Genesis (cf. Gen 2-3), was violated “in the beginning.” This appears still more clearly in the relationship of the Lord God with Israel at the time of Moses. The covenant made with the Chosen People at the foot of Mount Sinai (cf. Ex 24:3-8) contains as its constitutive part the Commandments” the Decalogue (cf. Ex 20; Dt 5). They constitute the fundamental and inalienable principles of behavior of every person in regard to God and in regard to creatures, especially to human beings.
According to St. Paul’s teaching in his Letter to the Romans, these fundamental and inalienable principles of conduct, revealed in the context of the covenant of Sinai, are “written in the heart” of every human being, even independently of the revelation made to Israel. The Apostle wrote: “When Gentiles who have not the law do by nature what the law requires, they are a law to themselves, even though they do not have the law. They show that what the law requires is written on their hearts, while their conscience also bears witness and their conflicting thoughts accuse or perhaps excuse them” (Rom 2:14-15).
vatican.va/holy_father/john_paul_ii/audiences/alpha/data/aud19861029en.html
Christ mentions those that the scribes and Pharisees mortally sinned:
Matthew 5:20 For I tell you, that unless your justice abound more than that of the scribes and Pharisees, you shall not enter into the kingdom of heaven.
Fifth Ecumenical Council - Constantinople II, 553 A.D. condemned the idea that there is not eternal punishment of impious men.
IX. If anyone says or thinks that the punishment of demons and of impious men is only temporary, and will one day have an end, and that a restoration ( apokatastasis ) will take place of demons and of impious men, let him be anathema.
fordham.edu/halsall/basis/const2.asp