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?
No, not that. Those are the negative consequences of not properly recognizing marriage as a lifelong covenant with rare instances of divorce.
The negative consequences are the fact that man and woman will not be recognized as one. Divorce will be difficult to deal with. Shared assets will be difficult to deal with. With marriage law, in cases of divorce assets can be split up properly. Those are just the most practical implications of not protecting the lifelong covenant between man and woman.
Well, part of that is the reason why legal marriage, or some means of assigning responsibility for people in the occasion of divorce is there. There are often kids involved when some people divorce, and you set the responsibilities in situations such as that, especially for making it responsibility for the divorced parents. In the same sense, you probably know who supports the wierd reinterpretations of marriage the most: the people who handle the divorce settlements.