sexual union vs pairing

sexual union and pairing both are nouns.

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

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

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

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

sexual union (noun)	pairing (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