adultery vs extramarital sex

adultery and extramarital sex both are nouns.

Word	Noun	Adjective	Verb	Adverb
adultery	Yes	No	No	No
extramarital sex	Yes	No	No	No

As nouns, extramarital sex is a hypernym of adultery; that is, extramarital sex is a word with a broader meaning than adultery:

adultery: extramarital sex that willfully and maliciously interferes with marriage relations
extramarital sex: sexual intercourse between individuals who are not married to one another

Other hypernyms of adultery include free love.

adultery (noun)	extramarital sex (noun)
extramarital sex that willfully and maliciously interferes with marriage relations	sexual intercourse between individuals who are not married to one another