married couple vs man and wife

married couple and man and wife both are nouns.

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

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

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

Other synonyms of married couple include marriage.

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