sexual relation vs sex act

sexual relation and sex act both are nouns.

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

As nouns, sexual relation and sex act are synonyms defined as:

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