same-sex marriage vs duo

same-sex marriage and duo both are nouns.

Word	Noun	Adjective	Verb	Adverb
same-sex marriage	Yes	No	No	No
duo	Yes	No	No	No

As nouns, duo is a hypernym of same-sex marriage; that is, duo is a word with a broader meaning than same-sex marriage:

Other hypernyms of same-sex marriage include couple, duet, twosome.

same-sex marriage (noun)	duo (noun)
two people of the same sex who live together as a family	a musical composition for two performers
	a pair who associate with one another
	two performers or singers who perform together
	two items of the same kind