There are only two sexes, change my mind:
Sure I'll take a crack at it. Lets first take the definition of what a biological sex is:
The biologic character or quality that distinguishes male and female from one another as expressed by analysis of the person's gonadal, morphologic (internal and external), chromosomal, and hormonal characteristics. -Farlex Partner Medical Dictionary
Interesting, lets analyse this definition. First of all on the surface it seems like this definition confirms only two sexes, after all it simply says that sex is the characteristics that distinguish between male and female, it says nothing about a third sex to potentially distinguish with. However when we look closer at the definition we actually notice it is quite vague. Sure it asks us to distinguish between only two sexes but the methods it asks us to distinguish with are many. It's gonadal, morphological, chromosomal and hormonal, why does it ask us to consider so many different features to determine something's sex? Surely if our biology lessons in high school should be believed we only need chromosomes to determine sex or perhaps you believe if they have a penis they must be male. Well, lets see what happens when we consider each metric individually to determine if there are only two sexes or not.
Gonadal Sex: This is the simplest definition of sex. If it has a penis then it must be a male and if it has a vagina than it must be a female so its very simple and easy right? This seems to say that there are only two sexes. Of course, however, we have to consider intersex people including people who are born with both a penis and a vagina. Now is someone who has both genitals a male or a female? Well if we only take gonads into account we actually cannot answer this question so this would have to be some other kind of third sex. The situation gets worse when you consider that even intersex people have variation, for example someone may have an external penis but internal ovaries that are generally not visible, in this case we might determine this person a male but they also have female gonads. The idea of using what gametes someone produces doesn't help us here either because a lot of intersex people are infertile and don't produce gametes, with both gonads and no gametes we cannot determine the individuals sex as male or female, this metric fails for only two sexes.
Morphological Sex: This is the external and internal characteristics of a sex so it's similar in definition to the gonadal sex but has extra components such as generally sexually exclusive features like facial hair and breasts. Even with this extra criteria we still have androgyny which are people who either have no sexually exclusive characteristics or have several different sexually exclusive characteristics in any combination. This is essentially someone who, at a first glance, is difficult to tell between male or female. It can even be difficult to tell in a medical examination room. The situation is further against using morphological sex because many intersex people are also androgynous. So even morphological sex does not sufficiently show there are only two sexes.
Chromosomal Sex: Ah, but we have chromosomes, the real indicator of sex. As we all learned the main thing that determines sex is whether you have an XX chromosome (female) or an XY chromosome (male) and so the case is solved. But of course we have to include all the other combinations such as XXY, XYY, X, XXX, and many others. Clearly just using those definitions indicates more than one sex so again chromosomal sex must not be sufficient to show that there are only two sexes.
Hormonal Sex: Finally we know there is estrogen and progestrogen which are female hormones and testosterone which are male so we can use levels to determine sex and distinguish between only two sexes. But this also doesn't work because these hormone levels are on a spectrum so it is very difficult to cut a line and simply say anything over this line is one sex and under the line is another sex. It's even worse because there are some (traditionally called) females that can have as high testosterone as males and so this too seems like a poor choice.
So have we done it? Have we proved there's more than two sexes?
I'm not convinced...
Because you know better right? You know that if someone has an XYY chromosome we can still call them male because they have a penis! And someone who may look kinda female we can still determine they're male because they have an XY chromosome. So clearly there must only be two sexes right? But wait, what are we actually doing here?
In the examples I gave all I did was pick a metric and showed some ambiguity and the response to determine the persons sex was to pick a DIFFERENT metric to resolve this ambiguity. If the chromosomal metric doesn't work (XYY) we then can switch to the gonadal metric (penis) to determine their "true" sex. What metric we pick doesn't matter as the definition after all asks us to consider everything all at once to determine sex so we just keep picking different metrics until something works.
But wait, isn't this actually a failure of classification? What if two metrics disagree about whether someone should be male and female, a classic example would be XX male syndrome which is someone who has XX chromosomes (and thus the their chromosomal sex is female) but has a penis and general male characteristics (their morphological sex is male). In this case we get a bit stuck because we are used to removing ambiguous metrics but in this case no metric is ambiguous, these metrics conflict. In the case of XX male syndrome we can't conclusively characterise someone as a male or a female and this really indicates that classifying everyone into two sexes doesn't work for everyone. It's not a complete category and there are variations and so are we done here.
No, there are still two sexes and then there are "abnormalities"
Ok sure. Now I could go into the fact these abnormalities account for about 2% of the population (same number as red heads) and thus are too big to be called simple exceptions but I'm not actually gonna go down that path. I'm gonna say that you can remove any set of outliers to classify your data into convenient categories and that is fine most of the time. The question we have to ask now is whether removing these outliers damages the quality of our data, does removing intersex people from the equation make our life easier?And that is the final crux of the issue. It's why people call others transphobic for asserting that there are only two sexes. It's because at the end of the day this isn't an issue about biology but it's an issue about classification and about what should be considered important and what should be considered simply an outlier. By claiming that all intersex people are simply abnormalities and removing them from our set we are overtly and bluntly stating that these outliers don't matter. That they SHOULD be ignored in our society, that they're unimportant and should be stricken from the record.
You have to decide whether intersex people deserve to be included within society, whether they deserve to be included within our classification and within our rules. This isn't a scientific proposition it is simply a moral one and if you choose to actively exclude intersex people from the very definition of sex, then I'm sorry, you are bigoted towards those people because you made that moral judgement.
At the end of the day...
It actually doesn't matter. The conversation of there only being two sexes is simply a way to invalidate transgender people. And it's true some transgender people probably fit in ALL metrics of being a different sex. We don't understand what makes someone transgender in the same way we don't understand what makes someone homosexual, they just are. We have two roads where we can accept that there exists different genders and that people can change their genders or we don't. When we don't we cause great pain to some people of the population, we allow people to murder them and we murder them indirectly through their suicide rates. At the end of the day I don't think it matters if they really are a different sex or not. I want to increase the overall happiness of society and it seems that acknowledging trans people does this, so I will do this. I hope you will to.
