In the United States of America is a married couple, regardless of sex, considered to be a single person by the government? If so, in what situations does the government not view them as a single person.
The modern rule is that individuals in a married couple are almost never considered a single person (ownership of real property in a tenancy by entireties in a few states mostly in the Northeast, is a narrow and rare exception).
Historically, considering a married man and woman to be a single person (a legal doctrine called coverture) was common place in the common law, but almost all such rules have been superseded.
Lots of the relevant law and legal history is considered in the answer to this question at Law.SE.