same-sex marriage vs twosome

same-sex marriage and twosome both are nouns.

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

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

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

same-sex marriage (noun)	twosome (noun)
two people of the same sex who live together as a family	a pair who associate with one another
	two items of the same kind