Why Extra-Marital Sex Is a Mortal Sin
Why exactly is sexual intercourse outside of marriage a mortal sin?
Sexual intercourse outside of marriage is a grave sin because in order to perform the acts that lead to the bringing forth of new life, the couple need to be united in a stable, permanent union that guarantees that the child to be born can be cared for and brought up for the service of God and neighbor in a consistent and reliable way.
The principal end of sexual intercourse is the procreation of children, and so the pleasure of the couple or the satisfaction of desire is only right if it is in the context of a union that can take care of the new life. This means that the man and woman have to love each other in so committed a way that they are willing to bear children to each other. To deal with sexual intercourse in any less careful way is a grave sin.