piece of tail vs sex act

piece of tail and sex act both are nouns.

Word	Noun	Adjective	Verb	Adverb
piece of tail	Yes	No	No	No
sex act	Yes	No	No	No

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

piece of tail: 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

piece of tail (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