You really don’t know what you are talking about. Every legal system in history has recognized marriage. There are serious negative consequences to not legally recognizing marriage at all.
Most legal systems in history have recognized slavery and polygamy, too...
I’d be interested to hear the negative consequences of not legally recognizing marriage at all. Is it the breakdown of the family? Fatherless households? Out-of-wedlock children? All those things we have now - with our legally recognized marriage?