Well, in opera there are so-called trouser roles, in which a young male part is sung by a woman. One of those is Cherubino in The Marriage of Figaro, and another is Octavian in Der Rosenkavalier. And of course there’s Hänsel in Humperdinck’s Hänsel und Gretel, but the character is a boy, not an adult.
I’ve also heard Prince Orlofsky in Die Fledermaus performed by both male and female singers.