groom-to-be vs adult male

groom-to-be and adult male both are nouns.

Word	Noun	Adjective	Verb	Adverb
groom-to-be	Yes	No	No	No
adult male	Yes	No	No	No

As nouns, adult male is a hypernym of groom-to-be; that is, adult male is a word with a broader meaning than groom-to-be:

Other hypernyms of groom-to-be include betrothed, man.

groom-to-be (noun)	adult male (noun)
a man who is engaged to be married	an adult person who is male (as opposed to a woman)