sexual union vs coupling

sexual union and coupling both are nouns.

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

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

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

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

sexual union (noun)	coupling (noun)
the act of pairing a male and female for reproductive purposes	the act of pairing a male and female for reproductive purposes
	a mechanical device that serves to connect the ends of adjacent objects
	a connection (like a clamp or vise) between two things so they move together