/scratching head/
1. Culture A has no written language.
2. Therefore, Culture A has no written documents for marriages.
3. Therefore, a married couple from Culture A cannot present any evidence of their marriage if they emigrate to Nation B.
I'm not sure what you're asking me here. Are you suggesting that a married couple from Culture A can uproot a cave painting of some kind that serves as their "marriage license" back in their home country, and bring it to the U.S. as proof of their marriage?
If married in a church there would be the church records to confirm that it took place on such and such date and who witnessed it.
Show me your sources for your claim.
Some people not being able to read doesn’t mean that their marriages do not have legal evidence.
Any such people would be an awfully small number, and they would be part of a larger culture.
Just saying that is happening isn’t enough. Besides, there you are again, with meaningless distractions, for meaningless fantasies.