Imagine you are walking in the park with a friend. Suddenly, your friend tugs on your shirt sleeve and says, “Look at that man over there!” You look in the direction your friend is pointing, but the only person you see is a boy of about 10.
“What, him?” you ask, pointing at the boy.
“Yes, that man right there,” your friend affirms.
Wouldn’t it strike you as odd that your friend chose the word “man” to refer to a 10-year-old boy?
I posit this scenario to demonstrate that the word “man” in English contains a number of features. Well, at least three: human, male, and adult. Any referent that does not meet at least these three characteristics cannot be called a “man”, barring special considerations. (By which I mean specialized, highly contextual scenarios where the typical usage of a word can be stretched. Imagine a father telling his son at his Bar Mitzvah, “Today, you are a man.” Is his son a man? In a specialized, cultural-context-dependent sense, yes. But would strangers walking down the street describe him as a man? Very doubtful.)
Imagine you’re again walking with your friend, who points to a woman and says, “Look at that man over there!” Again, there is a very jarring dissonance between the features of the word “man” and the features of the referent.
JWK is right in insisting “man” is historically gender-neutral, but meanings can shift over time.
So IMHO “man” is out as a term to generically describe all male humans, as in its common use it excludes non-adult male humans.
The word “male,” by contrast, carries no feature describing the age of the referent. However, it also carries no feature to describe the species of the referent–though if no species is mentioned, “human” does seem to be the default. For example:
“I saw four males.”
With no other context, most readers would assume these to be human males. However, in
“We went on this whale-watching expedition, and I saw four males!”
we’d assume the speaker meant four male whales. The feature [+ human] is not integral to the meaning of “male”.
As for English being “almost unique” in having no word for “male human being,” I think that’s a gap common to many Indo-European languages. Russian has “muzhchina” (‘man’), “mal’chik” (‘boy’) and “chelovek” (‘person’), but no word that encompasses all (and only) male humans. As far as I know. Native Russian speakers are welcome to correct me if I’m wrong here. Similarly, I think it would be weird in German to call a 10-year-old boy a “Mann” instead of a “Junge”, and it would certainly be odd to call a little French boy “un homme” instead of “un garçon.”
Oh, and I just want to add that I think it’s great that in many Romance languages, the word for “person” is gramatically feminine. I think of it as revenge against people who bull-headedly insist that inherently male words are in fact gender-neutral.