Interesting excerpts from cissexual radical feminist Catharine A. MacKinnon's 1979 book Sexual Harassment of Working Women I found while skimming through it:

Physical characteristics which provide indices of gender include internal and external reproductive organs and genitalia, gonads, hormone balance, and genetic and chromosomal makeup. Strictly speaking, in several of these physical senses gender is not immutable, merely highly tenacious. A transsexual operation, with hormone therapy, can largely transform gender on the physical level, with the major exception of reproductive capacity. But then many born males and females do not possess reproductive capacity for a variety of biological and social reasons. Aside from these characteristics, some evidence of physical differences between the sexes in the aggregate exists in the following areas: body shape, height and weight, muscularity, physical endurance, possibly metabolic rate, possibly some forms of sensory sensitivity, rate of maturation, longevity, susceptibility to certain physical disorders, and some behaviors at birth (irritability, type of movement, and responsiveness to touch). The scientific research stresses the wide, if not complete, mutability of even these differences by social factors such as psychological reinforcements, type of customary physical activity, and career patterns. Moreover, on the biological level, the sex difference is not a polar opposition, but a continuum of characteristics with different averages by sex grouping. (151-152).

She boldly proclaims that "biological sex" (gender, here) consists of a multitude of traits, almost all of which were malleable in the 1970s, and all of which will eventually be malleable in the 3rd millennium. Hilariously, this radical feminist has a more progressive stance on transsexuals than most of the LGBT (including the T!) community. To her, transsexual women can absolutely change their sex to become women. Yet due to the sex-gender confusion invented in the 20th century, many trans people and almost all of their allies don't think transsexual women are really women (just their brain is 🤪).

What hermaphrodism does to the concept of biological gender, transsexuality does to the concept of sex roles. The rigid exclusivity of each sex of the other is undercut in the clear presence of some of both. (154)

First, you'll have to excuse her antiquated language here; I don't think "intersex" had become mainstream until a couple decades later. Also, I find it interesting that she refers to "biological gender" and "sex roles" while today we call it "biological sex" and "gender roles." Maybe we should bring it back to emphasize how both are constructed rather than innate...

Anyway, on to the argument. So yes, the transgressive threat that intersex and transsexual people pose towards our patriarchal society's ideology of oppositional sexism needs to be reckoned with by feminists. Here the author just talks about transsexuality and gender roles, but the block quote above shows how transsexuals have the same effect on "biological" sex as hermaphrodism does. Serano talks about this in Whipping Girl when it comes to trans women, so at least it's still in the modern (serious) feminist consciousness. Society has a lot of mechanisms to minimize this transgression: arbitrarily define intersex as narrowly as possible so as to keep its official epidemiology low, insist that transsexuals do not change their sex, propaganda insisting that girls and boys are different, only allowing the most stereotypical feminine and heterosexual trans women to transition, etc.

Transsexuals experience a sense of sex identity cruelly trapped in a nonconforming body. (154)

Also, it appears she's been an outspoken trans ally in recent years as well, FYI.

Whatever the cause of this sense, it cannot be biological gender, since sex identity stands opposed to the body; nor can it be sex role conditioning alone, since sex identity is also opposed to that. (154)

Another own on the LGBT "community" (L stands for liberal here) - nowadays we have people insisting we were socialized one or the other gender, so we're not really that gender (they try to say this a bit more nicely though). But if that were the case, we'd identify with the gender that we were raised as! MacKinnon certainly expresses skepticism towards this line of logic.

The source of such a thorough rejection of standard sex role conditioning as well as physiology is obscure. But it is testimony to the power of the social correlation of sexual identity with physiology that, in order to pursue the desired behavior patterns fully, transsexuals consider it necessary to alter their bodies to accord with their gender identity. A final observation captures both meanings: first, gender identification may be better understood as a social definition of biology than as a biological definition of society, and, second, the power of that definition. (154)

Admittedly, I'm having a bit of difficulty parsing this, so my interpretation might be wrong. She's discussing gender socialization, gender identity, gendered physiology, and gendered behavior, all of which have social aspects. But then she makes the leap that transsexuals primarily seek to transition their gendered behavior/role(?), leading them to also transition their gendered physiology??? If this is what she means then I have some qualms. Wanting to be perceived as a woman and wanting to look like a woman (have secondary sexual characteristics that result from estrogen) are separate things, and they are also connected. But I'm pretty sure in the vast majority of cases, the conscious feeling of trans women is they want both, not that they want to be perceived as a woman and thus need to transition to attain this. Of course, subconsciously (and ultimately sociologically), that could be the case maybe... But it's not really possible to prove without a time machine or the ability to invent whole societies. The historical record of transgender people by definition does not predate the patriarchy. On the other hand, sex hormones and personal style do, so I'm inclined to think transsexuals would still seek transition even without sociological sex. Anyway, I wonder what her thoughts are now.

Commenting upon the justice of a proposed chromosome test for determining the femaleness of the transsexual tennis player Dr. Renee Richards, one woman observed: "I think nature is not always correct. . . . She looks like a woman, plays like a woman. She is a woman. Chromosomes make things scientific, but nature is not always a hundred percent correct." (154-155)

Nearly fifty years later and we're having the same debates, and the talking points have only regressed. 🙃