screwing vs sex act

screwing and sex act both are nouns.

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

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

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

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