making love vs sexual activity

making love and sexual activity both are nouns.

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

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

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

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

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