man and wife vs married couple

man and wife and married couple both are nouns.

Word	Noun	Adjective	Verb	Adverb
man and wife	Yes	No	No	No
married couple	Yes	No	No	No

As nouns, man and wife and married couple are synonyms defined as:

man and wife and married couple: two people who are married to each other

Other synonyms of man and wife include marriage.

man and wife (noun)	married couple (noun)
two people who are married to each other	two people who are married to each other