Divorce and death are two of the biggest reasons “marriage” matters from a financial perspective. Death not only for probate, but also death and dying medical decisions and visitation. Custody of children would be another big consideration.
I think if a couple is not actually married, then probate might give inheritance to a common law spouse, but my guess is if any federal taxes of the estate would apply, then the estate has to pay the tax, while if it has been a civil law spouse, the taxes would not be owed.
Regarding children, anyone who wants to remain a parent of step children, or similar circumstance, need to make sure wishes and legal work are done. I wouldn’t trust a judge on something so important.