are primarily straight women's fantasies. The transgression is of gender-assigned social roles, and the reward is heterosexual sex with the hunky hero (who initially saw her as a best friend.)
Sometimes the incidents of the story make this pretty clear. In Tamora Pierce's "Song of the Lionness," handsome prince Jonathan of Comte learns that his friend and page Alan is actually Alanna when a magical villain ... blasts her clothes off. Later "Alan", now Jon's squire, notices the prince making eyes at at pretty girl at a ball and takes advantage of their adjoining rooms to surprise him naked. The results can be inferred.