mixed marriage vs man and wife

mixed marriage and man and wife both are nouns.

Word	Noun	Adjective	Verb	Adverb
mixed marriage	Yes	No	No	No
man and wife	Yes	No	No	No

As nouns, man and wife is a hypernym of mixed marriage; that is, man and wife is a word with a broader meaning than mixed marriage:

mixed marriage: marriage of two people from different races or different religions or different cultures
man and wife: two people who are married to each other

Other hypernyms of mixed marriage include marriage, married couple.

mixed marriage (noun)	man and wife (noun)
marriage of two people from different races or different religions or different cultures	two people who are married to each other