The modern generic usage of the word “guy” exists because it was first a specific name, later generalized to mean “an example man”, along the lines of “every Tom, Dick, and Harry” or “regular Joe”.

“Guys…” They are doing something stupid, but a little bit funny.

“Man …” He’s doing something stupid. Not funny.

Am I right?

Frog guy: a guy who likes frogs

Frog man: a bog or swamp creature

guy, person and girl are all largely interchangeable