"Fornication" is sometimes used to translate the Greek word "porneia," which is a term in the OT related to idolatry. I wrote an article on that.

Note the complicating factors: unmarried sex is not a crime in OT law!!— and also slaves had no rights and couldn’t marry or refuse sex to masters, so saying "fornication" as a sex act is outlawed makes slaves unable to be Christian.

https://medium.com/belover/the-christian-sex-rules-were-a-misunderstanding-492d4dd861a5