sexual relation vs sex activity

sexual relation and sex activity both are nouns.

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

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

sexual relation: 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 sexual relation include sex, sexual activity, sexual practice.

sexual relation (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