mating vs sexual union

mating and sexual union both are nouns.

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

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

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

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

mating (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