"I am an adult man."
Translation:Jestem dorosłym człowiekiem.
Do you mean "Jestem dorosłym mężczyzną"? Because in the main answer, "Jestem dorosłym człowiekiem" everything is very clearly masculine...
And everything is masculine in "Jestem dorosłym mężczyzną" as well. But the noun "mężczyzna" seems feminine at first, true. And undergoes declension as if it was feminine. But it means "man", so logically, it has to be masculine. And takes masculine adjectives, possessives etc.