sexual union vs sexual practice

sexual union and sexual practice both are nouns.

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

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

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

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

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