If they’re smart roommates get documentation. At the very least you have the lease agreement they both should be on if they’re renting together. And if one owns and is renting to the other one then they better have a lease agreement. Siblings have documentation, they both have birth certificates that show they have the same parents and what order they were born in. Business partners that didn’t bother to create an business or at least a contract are in for a world of hurt.
You are talking “nice to have”, not requirements. And siblings may not have a father on their birth certificate, or they may have the wrong man listed.
Point is, I can make any legal contract I want, just as useful as a marriage license, without a state mandate or license.
Write up a contract and get married in the church. Since I don’t recognize the state’s definition of marriage, I don’t have to pay the tax penalties associated with it.