sexual union vs sexual activity

sexual union and sexual activity both are nouns.

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

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

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

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

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