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An article by the National Center for Transgender Equality defining nonbinary as a gender identity, and giving tips on how to be respectful and supportive of nonbinary people.
An article from The Trevor Project on the definition of gender and differing gender identities.
https://www.thetrevorproject.org/resources/article/understanding-gender-identities/
A research paper making the case for how a framework for a relationship and pornography scholarship could be organized into 5 different areas.
https://www.tandfonline.com/doi/full/10.1080/00224499.2019.1698003/