sexual union vs mating

sexual union and mating both are nouns.

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

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

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

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

sexual union (noun)	mating (noun)
the act of pairing a male and female for reproductive purposes	the act of pairing a male and female for reproductive purposes