One of the binaries most well-cemented in our contemporary society is that of the concept of “biological sex.” It seems relatively safe to suggest that most modern scholars are aware of the difference between sex and gender (the former referring to the body, and the latter to one’s self-identity). Nevertheless, though, while variations in gender are gaining broader representation – particularly in the form of transgender people, whose gender departs from their assigned sex – physical sex proves to continue to confound.
If you are like many others, you may take for granted various sets of universal binaries (binary here referring to a set of two). For example, dark / light, up / down, or male / female. These are considered to be polarized dichotomies; that is, they are viewed in a one-or-the-other framework. Something can be more towards pole A or more towards pole B, but it cannot have both qualities at once.
For some systems, this makes sense. Since darkness is the absence of light, for instance, it is logical that something dark cannot also be light. And since up is the opposite of down, one cannot go both directions at the same time. However, the relationship between male and female, both in the context of gender identity and biological sex, is what can be called a false binary or false dichotomy. An argument or statement using a false dichotomy presupposes a binary division or opposition between two things, when in actuality such a relationship does not exist.
Since we build our conceptualization of “male” and “female” from the foundation of biological sex, one must next logically question: in what ways is biological sex not binary?
To understand this, we should look closer at what biological sex actually is. As scientifically defined, the characteristic of “sex” refers to a physical type which usually has the function of a particular role in the reproductive process. To us, the familiar sexes are male and female. However, when asked to define what makes a person male or female, an average interviewee might struggle to produce a single factor. Instead, they may list a variety of characteristics associated with one or the other. “Long hair, breasts, uterus…” “Facial hair, testicles, deep voice…”
In a sense, such an answer wouldn’t be too off-base. Sex is not a single characteristic; it is a classification applied to a conglomerate of traits, which are divided into three groups: primary, secondary, and tertiary.
- Primary sex characteristics typically align with the sexual/reproductive system, and include sex chromosomes, reproductive organs, and external genitalia. This is usually what we think of when we say “biological sex.”
- Secondary sex characteristics are also physical and tend to result from hormone production. These include fat distribution and body shape, the development of breasts / mammary glands, growth of facial hair, bone structure and musculature, voice timbre, proportions, et cetera.
- Tertiary sex characteristics are varied and largely superficial, including things like wearing dresses and makeup, gendered haircuts, presentation and personality, creative or athletic interests, and so forth. 
If there are so many characteristics and factors associated with sex, how can one succinctly define it? A layperson, if they had gone through a cursory health or biology course during compulsory schooling, would probably narrow it down to primary sex characteristics. The assumption is that these (chromosomes, reproductive organs, genitalia), independent of outside appearance or cultural influences, are unbiased, divisible, and consistent. It is about “biological makeup,” is often the claim; “That’s a natural fact you can’t change.”
Unfortunately for the hypothetical layperson, this is actually a flawed understanding of the reality of sex. This is a more distinct instance of the above-mentioned false binary, and the division looks like this:
Set 1 | Set 2
XY chromosomes | XX chromosomes
has testosterone | has estrogen
sperm-producing testes | egg-producing ovaries
no uterus/menstruation/childbirth | uterus/menstruation/childbirth
external genitalia (penis, testicles) | internal genitalia (labia, vagina)
“MALE” | “FEMALE”
There are three beliefs that follow from the above assumption:
- An individual who has a single characteristic from either set must correspondingly have each of the remaining characteristics from that set.
- If an individual has a characteristic from one set, they must also lack the opposing characteristic from the opposite set.
- All individuals have possess only characteristics as expressed in one set, and no other characteristics or iterations thereof exist.
In simpler terms, any given person must have all of one set and none of the other, nor anything outside of the sets. If this were true, sexual classification would be neat and concise indeed! But that is a simplistic, uncolorful, and false world. In reality, the iterations of combinations of sex characteristics number vastly. To pretend otherwise is both inaccurate and practically inapplicable.
To begin to approach a more truthful understanding, then, let us return to sets – this time, the real ones.
Chromosomal Sets 
XX | XY | XO | XXX | 47XXY | 47XYY | 48XXXX | 48XXXY | 48XXYY | 48XYYY 49XXXXX | 49XXXXY | 49XXXYY | 49XXYYY | 49XYYY
Presence of Sex Hormones 
Oestrogen, progesterone, testosterone are typically all present in individuals at varying levels, while some conditions block hormone production altogether
Development of Gonads 
a) Ovaries develop, descend internally, and adhere to the uterus;
b) Ovaries develop, descend into the labium majus, and position like testes;
c) Testes develop, descend externally, and settle within the scrotum;
d) Androgen-insensitive testes develop, but do not descend;
e) Gonads develop to an incomplete extent across a range of presentations;
f) Ovaries are replaced with functionless tissue, develop into sterile gonads;
g) Seminal organs associated with testes develop, as does a pseudovagina
Uterine Presence 
a) Uterus is absent, in accordance with associated reproductive organs;
b) Uterus is absent, in contrast of associated reproductive organs;
c) Uterus forms completely (variety of possible shapes);
d) Fusing during development results in double uterus;
e) “Rudimentary uterus” develops without connecting to cervix and vagina
External Genitalia 
a) Genital tubercle elongates, shifts forward, and forms penile head/shaft, urethral folds fuse;
c) Genital tubercle shrinks, retreats, and forms clitoris, urethral folds remain apart and form labia;
c) Genital tubercle is ambiguous in size/position, urethral folds may be partially fused
Assigned “male” at birth | Assigned “female” at birth
Unassigned or assigned “intersex”
Assigned culturally unique “third sex” (of which titles vary per culture)
As you can see from the above, there are dozens of possible presentations of primary sexual characteristics, some of which (such as chromosomal makeup) we have no way of knowing without special testing. Someone with a vagina, then, may or may not have a uterus, typical levels of estrogen and androgen, or even two X chromosomes.
While they can occur naturally, from mutation, or due to birth or developmental anomaly, variations in chromosomal, hormonal, and reproductive systems are often titled “disorders.” Many identified chromosomal and hormonal disorders have unique names, and can be associated with other traits or conditions. These occurrences are usually identified because of their associated traits and symptoms, or else discovered in the process of some sort of testing. Sometimes they are treated medically, sometimes they are not.
Unlike variations in internal processes, though, variations in genitalia are typically identified at birth. Because a medical doctor at the time of delivery does not have the means to immediately examine, say, chromosomes or uterine structure, an infant child will be first assigned a medical “sex” based purely on the appearance of their genitalia. In many cases, the genitalia is consistent in form with either a vulva or a penis, in which case that child would be declared a girl or boy. However, not all genitalia adhere to these respects. If the genitals of a newborn appear to bear features of both forms, or if the makeup of the genitalia appears ambiguous enough to be indeterminate, the child is intersex.  Because intersex designation is such a big topic, you can learn more about it on its own post.
So, we know that sex is assigned at birth. A doctor designates a child usually one of two binary sexes, but in reality the characteristics associated with sex are multifaceted, broad, and often vary from our binary understanding of them. In accordance with actual representation, supposed “biological sex” is not truly divisible as factually binary at all. What’s more, some people develop physical characteristics at odds with their assigned sex due to hormonal or chromosomal disorders, some people cannot or choose to not participate in the reproductive role associated with their assigned sex, and some people grow to develop a gender which does not match their assignment, at which point they may or may not pursue any or multiple means of transition. If this is the case, what is the practical purpose of continuing to assign (binary) sex to individuals arbitrarily? Perhaps there is, in fact, no longer a place for such a structure in our society.
To learn more about how our binary gender system developed, check out my historical look-back.
 Wwnorton.com,. (2015). Chapter 4: Men and Women, Gay and Straight | Intimate Relationships: W. W. Norton StudySpace. Retrieved 30 September 2015, from http://www.wwnorton.com/college/psych/intimate-relationships/ch/04/summary.aspx
 Genetic.org,. (2015). What are X and Y Chromosome Variations? . Retrieved 30 September 2015, from http://www.genetic.org/Knowledge/WhatAreXYChromosomeVariations.aspx
 Users.rcn.com,. (2015). Hormones of the Reproductive System. Retrieved 30 September 2015, from http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/SexHormones.html
 Wikipedia,. (2015). Sexual differentiation in humans. Retrieved 30 September 2015, from https://en.wikipedia.org/wiki/Sexual_differentiation_in_humans#External_genital_differentiation
 Wikipedia,. (2015). Gonadal dysgenesis. Retrieved 30 September 2015, from https://en.wikipedia.org/wiki/Gonadal_dysgenesis
 Wikipedia,. (2015). Uterine malformation. Retrieved 30 September 2015, from https://en.wikipedia.org/wiki/Uterine_malformation
 Study.com,. (2015). External Genital Development in Males and Females – Video & Lesson Transcript | Study.com. Retrieved 30 September 2015, from http://study.com/academy/lesson/external-genital-development-in-males-and-females.html
 Tamar-Mattis, A. (2014). Patient advocate responds to DSD surgery debate. Journal Of Pediatric Urology,10(4), 788-789. doi:10.1016/j.jpurol.2014.03.019