Against Binary Conceptions of Sex
If you've read the groundwork previously laid, we can jump right into things. Sex is socially constructed. No, I don't mean gender (though it is, too) but biological sex.
All too often I see the refrain (often made tactically) that "gender is socially constructed and non-binary, while biological sex is not so and is binary."
I understand why this distinction is often put in these simple terms, but it is also worth detailing how this is, indeed, simplistic and how it can be harmful.
What is the reproductive system?
As outlined previously, actual material reality is messy and without templates, demarcations or "true" definitions. Take something like the reproductive system as an example:
That is a fairly typical depiction of a woman's reproductive system: uterus, fallopian tubes, cervix, vagina etc. Of course, reproduction requires substantially more than that to occur. Take out someone's lungs, heart and stomach and see how successful reproduction turns out. "Okay," someone may refrain, "the reproductive system relies on the circulatory system, the digestive system etc, but it can be defined by those organs solely or primarily functioned around reproduction."
Sure, it can be. That's probably quite useful at times. In other times, it is definitely useful to include, say, the pituitary gland or the thyroid and their effect on reproduction.
Even more broadly, it is rarely helpful to conceptualise an individual's reproductive system to include environmental factors (such as air or water quality) but that doesn't stop the actual material reality that these things are intimately connected. Without adequate air or water, human reproduction simply cannot take place, and so it is a required component of reproduction and the human reproductive system.
Similar to my previous mountain example, there is "just reality, all encompassing, all connected, universal reality." Where we decide to draw the lines is a function of what helps make reality comprehensible to us for particular purposes, not because those lines are in some form encoded into reality needing to be uncovered by us.
Males and Females
Take "female" for example. What makes a female? For millennia this was defined in terms of what we now call 'secondary-sex characteristics'. Breasts, vaginas, ovaries, etc. A "female" may be understood as that which gave birth or laid an egg. There are obvious flaws with using some of these markings as the determination of sex, particularly for purposes of human society. Rarely would we consider a woman who has a double ovariectomy to no longer be a "female." There is a clear tension in the fact that "male" and "female" as terms are intrinsically linked to concepts of "sexual reproduction" and yet are still applied to those who cannot sexually reproduce.
Many modern pundits, aided by the benefit of greater scientific knowledge, have now drawn definitions around chromosomes. "Males" have XY chromosomes, "females" have XX chromosomes. Nice and neat, right?
Well, these pundits don't normally hold this position for long, because obviously there are millions upon millions of "male" and "female" beings that have other chromosomes (say ZZ or ZW). And then you can get into X0, Z0, or why not XY' females and YY' males. There are X1X1X2X2 males and X1X2Y females. A male platypus has 5X and 5Y chromosomes!
So we can dive deeper still. Why is ZZ and XY considered "male" rather than, say, ZZ and XX? Scientists now link this to the relative size of the gametes used in reproduction. ZZ and XY produce the relatively smaller gametes to their counterparts of ZW and XX. Hence, males are those who produce the relatively smaller gametes and females those who produce the relatively larger ones.
There are of course entities that produce both the relatively larger and smaller gametes (e.g. hermaphrodites, gynandromorphism, intersex). There are also entities that reproduce through gametes that are the same size, with there being neither a larger nor smaller one (isogamy). While there are natural selection reasons for why gamete sizes converge towards just two, this does not mean that all sexually reproducing species only produce two sizes of gametes with certain algae, fungi and the drosophila bifurca producing three or more sizes of gametes. There are also gametes that, if no suitable mate is found, may simply reproduce asexually (parthenogenesis) which blurs the distinction between asexual and sexual reproductive species. There is sexual reproduction with 17,000, no 20,000, no 23,000 different "sexes" at play!
Evolution didn't result in a sudden neat switch from asexual to oogamous sexual reproduction
So to again emphasise: reality is not neat and tidy. It is invariably useful to talk of two broad categories of female and male, but this usefulness to us should not be confused for reality conforming to these ideas!
And as already foreshadowed above: even limiting our scope to just humans and just looking at "relatively larger" and "relatively smaller" gamete size does not solve all our problems. If someone simply does not produce gametes due to accident or biological happenstance, how should we group them?
Someone may again retreat from the above definition. It is not about whether they do or even can produce gametes of a particular size, it is about whether they share the phenotypical characteristics of the species that produces those gametes. E.g. you may not have ova, but you do have a womb and XX chromosomes like those that typically do produce ova.
And voila! We have come full circle back to secondary sex characteristics!
Why a binary at all?
I think that without a doubt, in the overwhelming majority of cases, it is useful to consider "males" and "females" as the two pairings required for sexual reproduction. However, it is inaccurate and rarely useful to emphasise this as a binary. Under a binary there are simple 1s and 0s. 1s are not 0s nor vice versa, and 1s are all 1s and 0s are all 0s.
In reality, even using relative gamete size to class species into male and female there is enormous variability between these classes. The differences between males, even looking very narrowly at sexual reproduction, vastly outnumbers their similarities. This isn't just at a macro level, wherein, for example, female neotrogla have penises that penetrate the male neotroglas' vaginas. The "relatively smaller gametes" vary in how they're produced, how they move, how they fertilise the "relatively larger gametes" which similarly come in all shapes, sizes and functions.
Talking about a "relatively larger gamete" producing specimen tells you remarkably little about the sexual reproduction of that species. Not how the newly produced being is grown, not if the female bears the young, not even if the gamete requires fertilisation by a "relatively smaller gamete" for reproduction to happen!
This is not to say that these definitions don't still have use. They do! The point is never that one definition is "wrong" and I am here to bestow upon the world the "correct" and "objective" definition. The point is to emphasise the necessary flexibility we need to have in order to communicate with each other and to comprehend a complex reality. There is a reason that studies of the Drosophila fruit fly have adopted terms like "metafemale" and "metamale" alongside "male" and "female" when studying its sex determination.
I think I've amply demonstrated that biological sex is complicated. It is not 1s and 0s. There are all sorts of blurred lines, and even in trying to zoom in to the most binary definitions possible we are left with gaps and confusions. The fact is "male" and "female" as concepts were not plucked from an objective understanding of the world, but predate modern scientific processes considerably.
A 14th century definition of "male" is given as "pertaining to the sex that begets young," as distinguished from the female, which "conceives and gives birth." They obviously weren't considering seahorses, pipefishes or all sorts of plants or other similar beings.
But it is from this sort of starting place that modern science picked up. It used the language already available to it to describe the world as we increasingly explored it, and hence imbued/tainted it with already existing beliefs and understandings of the world. Of course this happened, there is no other way.
The Politicisation of Biological Sex
What we see among some (rightwing pundits etc) today however, is a committed effort to maintain a binary of female and male. There is a drive to classify intersex beings as either "truly" female or male or to consider it somehow outside of a sexual binary altogether (akin to asexuality). This is not some neutral and objective pursuit.
When it becomes clear that a chromosomal definition of sex is not a neat binary, why feel the need to retreat to some more specific, more binary definition? Why feel the need to argue that the three gametes of certain beings "don't count" and there's "still an ova and then just different sized sperm"? Why does a gamete that can reproduce without fertilisation but still has that option need to be classed as "female" rather than some third thing?
But they aren't trying to create a language that helps us accurately describe this complexity. The impulsion is not from a curious scientific one. For some, the binary is the point.
They don't care about relative gamete size because of an interest in the natural world. They care about it because they want to class humans into two neat categories for social and political purposes. And they require the rigidity of a binary in order to uphold their own binary views of what men and women are and should be.
As soon as you bring in all the complexities and variations that relate to the sexes, from chromosomes to hormones to organs to genetics etc, it makes the nice neat categories of "men" and "women" seem a lot more complex as well. It is that, politically, that they cannot abide and hence the ideological pursuit of a binary.