making love vs sex activity

making love and sex activity both are nouns.

Word	Noun	Adjective	Verb	Adverb
making love	Yes	No	No	No
sex activity	Yes	No	No	No

As nouns, sex activity is a hypernym of making love; that is, sex activity is a word with a broader meaning than making love:

making love: sexual activities (often including sexual intercourse) between two people
sex activity: activities associated with sexual intercourse

Other hypernyms of making love include sex, sexual activity, sexual practice.

making love (noun)	sex activity (noun)
sexual activities (often including sexual intercourse) between two people	activities associated with sexual intercourse