With your "marry anyone and divorce them anytime you want" rule, just marry everyone you do an illegal deal with. You can't be compelled to testify against your spouse. Divorce them after the deal, and the illegal acts are still under priveledged communication rules.
Abandon all that, and telling your wife that you hate the boss at work, can be used against you when they find that the boss was run over at work. So you either have to change millions of laws, or break the idea of "marriage" completely.
Which do you find preferable? Why?
Most people find it easy that they can have a single contract with no modifications or negotiations allowed (though separate property contacts, commonly called pre-nups, are allowed, though in practice, rare). The millions of laws written around marriage work together to define it in a contractual, legal, financial, and societal context. It may not be perfect, but it's better than abolishing a legal recognition of any relationships.
The law could very simply state (all laws regarding marriage are null and void. from this day forward the laws are as follows....
Yeah, and so the one law passed to do that is in what jurisdiction? Federal? They don't define "marriage" now, but put rules on it based on what the 50 states decide. So at a minimum, you'd have to have 51 states (DC is a "state" for most purposes), plus the feds, and get that 52 law bundle passed at the exact same instant for that to work. Plus, the "laws" in many places aren't laws, but regulations and administrative rules. Your "simple" law would have to change the IRS code, and hundreds or thousands of other federal regulations with weight of law. And the countless local rules on marriage. State law in Texas allows a minor to drink, under the supervision of an adult. So a 21 year old married to a 19 year old, can buy drinks for, and hand drinks to the "under-age" drinker. But that's not the same everywhere. Thousands of little things like that would make a massive change to the legal burden of formerly married people, once you abolish marriage, in the way you state.
You obviously don't even understand that, let alone have an opinion how it would work after. How does your one simple law fix under-age drinking law in Texas (a state matter) and the IRS code (not coded into law), at the same time?
"One simple law" change for multiple independent jurisdictions. With no understanding of law, or reality.