sexual union vs conjugation

sexual union and conjugation both are nouns.

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

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

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

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

sexual union (noun)	conjugation (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 making or becoming a single unit
	a class of verbs having the same inflectional forms
	the complete set of inflected forms of a verb
	the inflection of verbs
	the state of being joined together