I'm still reading this with Serano's new book in the back of my mind, and especially with Wittig's essays in the front of my mind, since Butler deals with her "materialist feminism" quite a bit. For Wittig, all gender, "male" and "female" is part of a single construct which patriarchal oppression created-- not vice versa (she equates sexual difference theory a la Irigaray with blaming the victim). Gender is how patriarchy marks bodies so that they are processed through the usual undisturbed channels of socialization and heterosexual society operates smoothly. This is a material semiotic process in which material bodies are made to serve as material signs, which is why she refers to "marking" rather than "construction."
Now, despite Wittig's avowed materialism (of the Marxist sort), her position on the origin of all gender in oppression does not allow Serano's distiction among "gender expression," "unconscious sex" and "biological sex" to be a real distinction. Serano introduced that distinction to do justice to her experience of a painful dissonance among her unconscious sex (female), gender expression ("tomboyish"), and biological sex (male). For Wittig, perhaps Serano was insufficiently processed and marked by the heterosexual order, resulting in her transsexualism. I don't know what W. would say to it. The more interesting question Wittig might raise to Serano is, "The fact that you pass for [in the French sense] a trans lesbian, is more evidence for the fact that lesbians are not women." If one who mostly escapes heterosexual marking as "female" is a lesbian according to Wittig, what about one who mostly escapes heterosexual marking as a "male?" This raises the question: if lesbians are not women, according to Wittig's origin-of-sex theory, they cannot be men either. And those fugitives from sex who escape the usual channels that mark them as one or the other-- are they all fundamentally lesbians? That seems wrong. What about gay men? By her logic they are not really men in the heterosexual sense, which is the only sense in which "man" makes sense to the dominant social order that creates men. And what about a transsexual? Wittig's theory anticipates the possibility of queer transgendered people; in fact, the theory suggests that some element of transgenderism is inevitable when one becomes queer.
Elizabeth D. Däumer's paper "Queer Ethics; or, the Challenge of Bisexuality to Lesbian ethics" goes right to the heart of this matter. It asks whether a lesbian, on Wittig's theory of hetero. marking as sex difference, need be female, and whether a lesbian relationship (which is the process that erases hetero. sex marking, it seems) is necessarily between two females. Clearly, it is on the other hand necessarily ~not~ between two women, because the category of woman is an effect of heretosexual marking. Wittig, and certainly Butler in G.T., wants to say that sex is a social designation as much as gender. So, as Däumer considers, Wittig's theory may entail that the lesbian relationship crosses all previous hetero. markings and thus all sexes and genders. It's likely that the word 'lesbian' is being applied beyond any recognizable usage here. Whatever name you wish to put to this sort of relationship, its content is a sort of anti-hetero. sexual/gender ethics which seems to me to herald the possibility of a truly universalist androgynous society.