Different countries and cultures have their own protocol for marriage (weddings).
Yup - just ask Jacob, who woke up the next morning WITH THE WRONG GIRL!!!!
If a man and woman live together as man and wife without being married,how is it sin?
It's NOT. When they "join flesh", they're MARRIED, and as long as they remain monogamous, there's no SIN involved.
I find nowhere in scripture where they must have a license.
The "wedding ceremony" is PRIMARILY the execution of a LEGAL PROCESS, who's function is related to legal access, ownership of property, and legal status of any resulting CHildren. Ideally, the couple goes through the Ceremonial hoopla, and THEN goes off in private, and gets MARRIED. When there's NO marriage contract, but there are "Material assets" when/IF the couple separates, then it gets legally messy. many states in the U.S. have "Common law" statutes, which stand in place of a marriage contract when things come apart.
AND, of course, getting a "LEGAL DIVORCE" has no effect on the Spiritual "State of marriage" that exists, it's only a "Legal" status.
If they are faithful to each other,is there fornication taking place?
Nope. as long as they're true to each other, everything's spiritually O.K. but LEGALLY it can get messy.
Man makes laws,but how would God look at it,
When you JOIN FLESH - you're married. If you join flesh with somebody else, you're in Adultery.
Simple as that.
In the OLD Testament, NOBODY was indwelled by the Holy SPirit, and they were "stuck" with their human natures with all it's appetites and lusts. Multiple marriages were allowed (Probably to prevent people from murdering the unwanted spouse to get free of him/her. (The hardness of their hearts Mat 19:8).
Biblically, marriage is intended to be
one MAN, and one one WOMAN - for LIFE. And given the "Moral condition" of U.S. Society, a significan number of couples who have "Weddings", have actually been married already, and are probably already in multiple adulteries.
It doesn't matter what this or that church system says, or what the "Pope" decides to "bless", or what the LBTQXYZ and "Trans" Idiots want to make "normal".
Biblically:
ONE MAN, and ONE WOMAN for life. For me it's been 58 years, and we did it in the "Normal fashion" back in '65. Result: 5 children (1 didn't make it), 13 grand kids, and 3 great grand kids.