“It is much more than that, a marriage has to be known as legal, for instance for military pensions and survivor pensions.”
Your assumptions are not correct.
Military pensions are a percentage of base pay for time served over 20 years. Not allowances (dependants are an allowance calculation).
Survivor benefits are paid into by the retiree. If the retiree and the spouse decide not to pay into it... it doesn’t exist. It’s something like term life insurance.
Medical insurance is also paid into.
I’m not talking about whatever the new policy is, I meant in our history and the military was just an example, people have had to know if a person was married to the other person for a lot of reasons.
Licenses long predate America, Washington and Jefferson paid for licenses, Revolutionary widows had to prove their marriage to a dead soldier.