crossbreeding vs sexual union

crossbreeding and sexual union both are nouns.

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

As nouns, sexual union is a hypernym of crossbreeding; that is, sexual union is a word with a broader meaning than crossbreeding:

crossbreeding: (genetics) the act of mixing different species or varieties of animals or plants and thus to produce hybrids
sexual union: the act of pairing a male and female for reproductive purposes

Other hypernyms of crossbreeding include conjugation, coupling, mating, pairing, union.

crossbreeding (noun)	sexual union (noun)
(genetics) the act of mixing different species or varieties of animals or plants and thus to produce hybrids	the act of pairing a male and female for reproductive purposes
reproduction by parents of different races (especially by white and non-white persons)