"Mainstream gender" as a system of oppression.
Imagine mainstream gender as a line. On one end of the line is "man," and on the other end is "woman" – this is the gender binary. The connecting line itself is representative, loosely, of "non-binary" gender. (I think this image is suitable, and if others disagree, I think that's more representative of how arbitrarily the binary is defined, rather than this image of it.)
This gender spectrum, dubbed "mainstream gender" because it's what the institutions of society use, only applies to certain groups – the group that controls it, really. White people.
Black people cannot access mainstream gender because they are barred from it via hyper-masculinization that borders on animalization. Black men are viewed as hyper-aggressive and predatory. To the white masses, the concept of "soft" Black masculinity is as real as the transatlantic slave trade (i.e., vague notions of it existing, but no personal experience with the implications, so it floats in the periphery until it's brought up again by a Black person). Black women, regardless of how much they lean into "traditional" femininity, will never be viewed as feminine. Black women will always be viewed as loud, aggressive and "man-ish," the latter contributing to transphobia that impacts both black trans women and black cis women.
By contrast, East Asians are hyper-feminized. East Asian men are stereotyped as soft and small and are portrayed as having little sexual appeal (and those who are viewed as sexually desirable are seen as exceptions to the mainstream; sexy despite their femininity). East Asian women are equally unable to escape femininity and are hyper-feminized to the point of infantilisation, both of which contribute to hyper-sexualization and fetishization. The hyper-femininization of East Asian people also lends itself to transphobia targeted at East Asian trans men (recall: the 2016/17 obsession with smol bois).
But not only is mainstream gender a tool of white supremacy, it also (obviously) is inherent to patriarchy (or, patriarchy is inherent to mainstream gender).
The binary is defined and propagated by men. So while all gender rules are arbitrary and constantly changing, "man" is always strictly defined, and the relief of "man" becomes "woman." This is evident both in masculinity being viewed as "gender neutral," as well as the sense of authority men feel over women's gender, commenting on makeup, how women dress, etc. As a result, the gender "woman" is inherently centered around "man."
In de-centering men, lesbians lack the rules to adhere to mainstream gender. That is why lesbianism is often described as a gender experience because lesbians operate on a new gender spectrum, specific to lesbians.
However, different from race, sexuality is not a visible barrier, and it can take work to exit from mainstream gender. Terf "lesbians" are not lesbians because they cling to the relief of "man" to define their gender; they're gay women. Similarly, white lesbians who prioritize their whiteness will continue to exist within mainstream gender. This also explains why white non-binary people are "like that." While they do not adhere to the binary of "man" and "woman," in participating in the system of whiteness, they remain within the confines of mainstream gender.
