On a practical level, what does this mean for things like survivors’ insurance in Social Security, etc.?
Simple: Equal fractions.
It means polygamy would create the best tax loophole ever seen. Legal marriage in the US and a religious understanding of holy matrimony are entirely distant things. The state has completely separated sex from marriage. They’ve taken away permanence through no fault divorce. Legal marriage is simply a financial arrangement between parties that allows for special privileges on a temporary basis. So why not marry to avoid inheritance laws, to sell a business, or whatever works.
I was single when I retired.
A few years later I got married; and for her to receive benefits from my pension; I had to pay back into the retirement for the years I was single and retired.
I would imagine that each extra wife (husband?) would require and additional monetary input to your retirement/SS account(s).