pairing vs sexual union

pairing and sexual union both are nouns.

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

As nouns, pairing and sexual union are synonyms defined as:

pairing and sexual union: the act of pairing a male and female for reproductive purposes

Other synonyms of pairing include conjugation, coupling, mating, union.

pairing (noun)	sexual union (noun)
the act of pairing a male and female for reproductive purposes	the act of pairing a male and female for reproductive purposes
the act of grouping things or people in pairs