The harness is NOT why males do not play Peter Pan.
One word: TRADITION!
There's many factors going into the reason. Child labor laws, a fully grown woman looks and sound like a boy more so than a fully grown man, etc.
Concerning the Charlap/Leigh musical, the songs were written with Mary Martin in mind. That's why with this musical (along with tradition), a woman is cast.
There are other musical adaptations of the Peter Pan story where a man plays Peter. The music is very much so better suited for a male voice.
Nothing to do with harnesses.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle