The word fornication is a synonym for prostitution, and is not a synonym for adultery as you claim.
A biblical marriage is simply an agreement between two people to shack up together and doesn't require a legally signed marriage contract or a wedding celebrant or witnesses.
And Jesus condemned ALL remarried divorcees to hell unless they remain celibate and repent their adulterous marriages (Mark 10:11-12 Matt 5:27-30). And it's your choice if you don't believe what the bible says................
Fornication comes from the Greek word Porneia. This is even where we get the word pornography.
Because porneia ( fornication ) is more comprehensive that adultery is why Jesus gave fornication as scriptural grounds for divorce.
Fornication can include having sex with an animal as scriptural grounds for divorce.
Please see Matthew's parallel account found at Matthew 5:32 and Matthew 19:9.
P.S. 'Biblical hell' is just mankind's temporary grave for the dead - Acts of the Apostles 2:27.