The church knows those sins are
grave matter. It means that they
can be mortal sins provided the other conditions are met. The degree of culpability is something that is difficult to know for sure; a good confessor can
usually figure it out but often it falls into a grey area.
The Church is very clear on this:
1857 For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.”
1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: “Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother.” The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God’s law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart133 do not diminish, but rather increase, the voluntary character of a sin.
(Catechism of the Catholic Church)
You absolutely do not know the state of someone’s soul. Let’s say someone frequently absent at Mass. You have no way of knowing whether they had good reason. For example I rarely go to Mass in my parish. I’m a Benedictine Oblate and I go to Mass at the monastery; others may prefer a neighbouring parish due to more convenient Mass times, or it’s closer to an elderly relative and they help them get to Mass. Or that person may have a young child or an elderly parent to care for, or themselves may have a health issue.
The person having sex outside marriage may have mitigating circumstances, such as pressure from their partner. Or they may be having difficulty accepting that teaching and need time (and God’s grace) to come around.
You simply do not know from appearances what is going on inside someone’s soul. So even casually saying “people who miss Mass are in a state of mortal sin and shouldn’t receive” may prove highly insulting to a person having serious issues with elder care that you don’t know about. They may deeply wish to be at Mass but perhaps every other weekend they alternate care with a sibling, or have no siblings to assist and can’t make it to Mass.
The person having sex outside marriage may deeply wish to stop but is having great difficulty doing so in spite of frequent confession. Difficulty resisting temptation, fear of losing their partner, etc. They may be working weekly with their confessor to try to find a solution. But the seal of the confessional ensures you wouldn’t know that.
You simply don’t know, which is why, as my wife likes to say, it’s always best to “shut the flippin’ up”. We have no business being busybodies.
Instead of being rule-filled when it comes to others, we should be grace-filled and treat every person, every sinner (of which we are one) as the Rule of St. Benedict says “as if Christ Himself was present”. Because He is. Everyone of us has an image of Christ in us. That is the light in which we should deal with persons, and when it comes to sin, it is our own conversion we should be working on.