As I commented in someone else's journal as part of a related discussion (note that I totally threadjacked a post on property rights to get into this)
So "separate but equal" is good enough? Are you at all familiar with this study by the GAO?
It finds "1049 federal laws classified to the United States Code in which marital status is a factor."
The laws are broken down into 13 categories. "The order of the categories is not significant, except that the first four are those in which
marital status is most pervasive, and are the largest."
Social Security and Related Programs, Housing, and Food Stamps
Federal Civilian and Military Service Benefits
Employment Benefits and Related Laws
Immigration, Naturalization, and Aliens
Trade, Commerce, and Intellectual Property
Financial Disclosure and Conflict of Interest
Crimes and Family Violence
Loans, Guarantees, and Payments in Agriculture
Federal Natural Resources and Related Laws
Good luck to anyone trying to "draw up their own roughly equivalent legal contracts on their own". Of course, many of the benefits of marriage come from outside the marriage itself, making the idea of recreating a marriage-like legal framework on an ad hoc basis all the more absurd.
That isn't 1049 marital benefits or privileges, it's 1049 related laws but a lot of them relate to benefits or privileges. It's an older study, but well worth the read.