Genesis 34:4-7

Genesis 34:4-7 (KJV)
4 And Shechem spake unto his father Hamor, saying, Get me this damsel to wife.
5 And Jacob heard that he had defiled Dinah his daughter: now his sons were with his cattle in the field: and Jacob held his peace until they were come.
6 And Hamor the father of Shechem went out unto Jacob to commune with him.
7 And the sons of Jacob came out of the field when they heard it: and the men were grieved, and they were very wroth, because he had wrought folly in Israel in lying with Jacob’s daughter: which thing ought not to be done.

Fornication Is A Grievous Sin

Sex before marriage is unlawful in the eyes of God. The very act defiles a woman. It violates her chastity, she is keeping for marriage.