free love vs extramarital sex

free love and extramarital sex both are nouns.

Word	Noun	Adjective	Verb	Adverb
free love	Yes	No	No	No
extramarital sex	Yes	No	No	No

As nouns, free love and extramarital sex are synonyms defined as:

free love and extramarital sex: sexual intercourse between individuals who are not married to one another

free love (noun)	extramarital sex (noun)
sexual intercourse between individuals who are not married to one another	sexual intercourse between individuals who are not married to one another