Many of the rights discussed in this context, like inheritance and hospital visitation and medical decision-making, are rights that have been “incidentally” tied to marriage. That is, they are rights that seemed to be the easiest to administer by using the state of matrimony as a convenient criteria. But many of these rights could reasonably be extended to other relationships. There is nothing essential about marriage that would prevent their extension. Therefore when examples of these rights are presented as evidence, there are two possible ways to grant these rights. One is to explicitly extend them as appropriate to the specific right involved. The other way is to extend the definition of marriage to include other relationships.
While method #2 does appear to be temptingly simple, as opposed to the right-by-right consideration of method #1, I think method #1 is the preferable one. It allows the various rights to be optimally extended to just the relationships that are most appropriate for those rights, and avoids the unintentional consequences of the wholesale redefinition of marriage everywhere in law.