It is a mortal sin - a grave sin against the Sixth Commandment. You have heard that it was said, You shall not commit adultery.
Fornication is carnal union between an unmarried man and an unmarried woman.
The Catholic Church has always led a countercultural stand regarding pre-marital sexuality. The Church promotes the understanding that sexuality and commitment must go hand in hand. The Church demands that a man and a woman make the marriage commitment first and then engage in the sexuality within the marriage. Sexual freedom is a freedom to tragedy and destruction. Sexual freedom is not a freedom but a slavery in its real sense.