sex act vs sexual activity

sex act and sexual activity both are nouns.

Word	Noun	Adjective	Verb	Adverb
sex act	Yes	No	No	No
sexual activity	Yes	No	No	No

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

sex act: sexual activity between individuals, especially the insertion of a man's penis into a woman's vagina until orgasm and ejaculation occur
sexual activity: activities associated with sexual intercourse

Other hypernyms of sex act include sex, sex activity, sexual practice.

sex act (noun)	sexual activity (noun)
sexual activity between individuals, especially the insertion of a man's penis into a woman's vagina until orgasm and ejaculation occur	activities associated with sexual intercourse