Contracts can never bind third parties. Marriage can.

Can you elaborate, please?

But if you call it a contract fine - the point is that insofar as it needs to be special/different from all other contracts, it needs to have rules. Otherwise at the extremes you could just put those things in an employment contract, call it a "marriage", and get away with anything.

Well, I think employment contracts should be laxer too. It also concerns consenting adults. I'm not sure what "rules" means here - contracts are collections of rules, or they can be. If you're angling at the idea that contracts should be restricted by the government, then there are some arguments for that. Particularly if the contract is temporally extended over a dramatic span.

Sure, sure, but if that's all it is, there's no reason to privilege it. You wouldn't need a government-mandated default[.]

Whether or not you'd need a government-mandated default contract, there is one. Otherwise, you encounter the problem of those who have not chosen contracts, and society does not like that ambiguity.

Punishing people for the sin of trusting the government appeals to me on one level, but I'm not convinced it's good public policy.

Me too. :P

Seriously speaking, it's not punishing people for trusting the government. Congress is an institution which can change its mind - everyone knows that, it's what democratic government has been like for decades. I don't mean that Congress simply does change its mind, I mean what it is as an institution entails that it can change its mind: everything the people do is subject to the people's review. Congress has the power to make and unmake laws.

There might be pragmatic arguments in favor or against Congress changing its mind, etc. I'm not arguing that. I'm just pointing out that laws are made by Congress. That's all. If you expect Congress to suddenly lose the power to make and unmake laws, you may be surprised.