There must be other creative ways around taxes/inheritance tax without using “adoption.”
For example: Why couldn’t one guy simply write the other guy into his will? Or why couldn’t they just add each other’s names to each other’s accounts and investments?
A person can leave an inheritance to a spouse without paying estate taxes on any of it, and there may be life insurance provisions based on a second-survivor policy (it's complicated, but basically both partners are insured under one policy). It may be necessary to have both "partners" listed as spouses, and to have the adult "child" listed as the legally adopted child of the second partner, to deal with potential legal challenges from other children from a prior relationship.
I don't pretend to know all the details of this, but it is not unusual for people to come across situations like this in financial/estate planning.