sexual union vs sex activity

sexual union and sex activity both are nouns.

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

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

sexual union: the act of pairing a male and female for reproductive purposes
sex activity: activities associated with sexual intercourse

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

sexual union (noun)	sex activity (noun)
the act of pairing a male and female for reproductive purposes	activities associated with sexual intercourse