"Yes, I am a man."
Translation:Tak, jestem mężczyzną.
"mężczyzna" is one of the strange massculine nouns that ends with "a" - it declines like a feminine noun, but has masculine adjectives, pronouns and verb forms
It is not the only one, there are more words like that.
"Mężczyzna” isn't any exception to rules of declination. It declines exactly like other masculine nouns end with "-a", e.g. "dentysta", "specjalista", "patriota". They also decline like feminine words and have masculine adjectives etc. "Mężczyzna" is a strange word because this is the only masculine noun with ending "-yzna". Other words with this ending are feminine.
Yes, it belongs to this small group of nouns denoting men despite ending with -a.