While the specific phrasing you used is often associated with adult or derogatory slang, there are several high-quality academic and analytical papers that explore the representation of characters in anime from a scholarly perspective.
: Published by the Illinois Open Publishing Network, this article analyzes works like Stop!! Hibari-Kun! and No Bra to understand the appeal of gender-ambiguous characters to both male and trans-feminine readers.
: An exploration on Sabukaru Online of how Japanese media allows for a wider range of character archetypes and "gender spicing" compared to Western media. Important Terminological Note
: This paper, available on ResearchGate, examines gender transitioning in anime and argues that some subgenres may reinforce binary gender concepts rather than being purely progressive.