Animorphs, any characters, morphing into the right gender for the first time, but it's still not your own body
Animorphs, any (Yeerk) character, Yeerks just don't quite get gender

Order of the Stick, Vaarsuvius, what's under Vaarsuvius' robes isn't nearly as interesting as what's contained in their arcane power filled mind, but alas, some people are obsessed with such insignificant matters