[masaegu]

That is a good question. If you say just おとこ, it sounds very rude to the Japanese ear. It would sound like you are talking about a crime suspect. The same thing with the word おんな. We just don't use those two words by themselves very often.

おとこのひと = man

おとこのこ = boy

おんなのひと = woman

おんなのこ = girl