Fornication was never normal, it was made up after marriage,fornication never works in a society, even from an evolutionary perspective, Marriage was the norm since the beggning of time, it produces the most stable familys and societys, allows people to track lineages, is most natural for people in terms of time work energy and makes most sense for both genders.Fornication was always normal to begin with. Jair because someone invented a religion and prohibited people to do it by force doesn't mean it is not normal
You can even see in alot of animals that have mass monogamy, that is their natural way, marriage or being deidacted to one person sexually is even found in nature.
Your assertion fornication was normal needs to be backed up with evidence, and giving me history of greeks and romans who came thousands of years after the first humans is not evidence.