Which of the following is an example of a "must" societal norm for marriage in the United States?
Married individuals must live together.
Marriages must be specifically religiously sanctioned.
Married individuals must have children.
Marriage must consist of two people.
I think the only qualification is likely that it requires no more nor no less than 2 people. There are so many gender classifications that this is likely all that is left. All the other choices are maybes.