If they want to they can make a new kind of arrangement without any mention of marriage that would be exactly what you mention, a legal contract for visitation and inheritance. The document should work for siblings and dogs or caretakers. The fact is, they are after the Marriage title. They are actualy after the Marriage Institution. So look at it this way: We, the people at Marriage Institution, want to keep our institution running. You can open your own if you so desire, but don’t knock down my time honored institution.
If the government sanctions this redefinition, it is the new norm. Government is not your quiet neighbor. It is the creator of standards, or breaker thereof.