Just a holding link for now on the civil marriage / partnership fiasco.
(We need to rehabilitate the word normal not being a value judgement – normal = most expected situation, abnormal = valid / legal / accepted variations to the norm. Previously. And before that.)
Consider the starting point Q – why would the state concern itself with a relationship between two people. A – when there are innocent third-parties involved, not party to the original “contract” – future children. Protection of children as individuals in their own right, and as part of the state’s future.
Otherwise parties are able to agree any kind of contract they like with each other, which can include for example pre-nup arrangements for when that contract is deemed to be broken. You can just get on with it, with or without the lawyers, with as much or as little contractual formality as you like .
The constitution of the state sets standards for values of that constituency. Any legal contract in that state, involving the state or not, must meet those values. A contract, again involving the state or not, can adopt additional values provided they do not conflict with the state standard values.
Not surprisingly traditional values arising from religious and ethical traditions are often embedded in state constitutions.