sex act vs sex activity

sex act and sex activity both are nouns.

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

As nouns, sex activity is a hypernym of sex act; that is, sex 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
sex activity: activities associated with sexual intercourse

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

sex act (noun)	sex 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