What exactly is marriage? Is it a religious institution, or is it a civil institution? What is the purpose of marriage?
Points to ponder:
#1 In the USA, it is possible for two adults to live together without being married.
#2 In the USA, one adult can grant another adult Durable Power of Attorney.
#3 In the USA, one adult can grant another adult Power of Attorney for Health Care.
#4 In the USA, an adult can make out a will which designates another person as executor. The will can also designate a beneficiary.
#5 There are nations in which polygamy is legal.