Fornication is whoring around. After Shechem had sex with Dinah, they did not just commit to each other for life and call themselves married. Shechem's father realized that Dinah's father had to give her in marriage. Their fornication (or his rape of her) was not marriage.
After the whole story was done, Simeon and Levi asked, 'Should he have dealt with our sister as a harlot?'
If a girl lost her virginity and was married off as a virgin and discovered not to be one, she could face the death penalty for fornicating in her father's house.
Sex is supposed to take place within the confines of marriage, and the types of marriage God accepts, not two men or two women, not between close forbidden relatives,