Just to clarify, neither of the parties is married.
So as I understand it they can not be commiting adultary. You are correct, they are not committing adultery. However, there is more than one sin contained within each commandment. Fornication is also a mortal sin against the 6th Commandment. In fact, because sex is sacred, all sexual sins are mortal sins.
Other than this issue** they seem to be good Catholics, the children are baptised, they go to church and confession regularly.**
Not sure if you are referring to the parents or the children here - if the parents go to confession regularly, they must not be confessing their living in sin! If so, they are committing additional mortal sins by making bad confessions.
What is the sin if they love each other, and bring up a family without getting married ?
**What is the sin? Are you serious? **
Maybe it could be considered a minor error rather than a deadly misdeed, if it has to be considered a sin. To be living with someone in a sexual relationship without being married a minor error??? You can’t be serious!
It seems too harsh a punishment for what is really a technical issue. **If they continue to live in this manner, they are seriously endangering their eternal lives. Objectively, they are in mortal sin. **
Would you not agree ? Clearly, I do not agree with you. You need to read the relevant chapter in the Catechism of the Catholic Church.
The Catholic Church teaches that human life and human sexuality are inseparable. Because Catholics believe God created human beings in his own image and likeness and that he found everything he created to be “very good”, the Catholic Church teaches that the human body and sex must likewise be good.
Surly sex before, and therefore outside marriage, can only be a celebration of Gods design ?