Biological sex

Biological sex has two common meanings: (1) the two mating types of sexually reproducing organisms, and (2) the range of sexual phenotypes of individuals belonging to a sexually reproducing species. The sexual phenotypes are defined by numerous biological markers in humans and other organisms: sex chromosomes, genitals, gonads, gametes, hormones and secondary sex characteristics. The two mating types are "female" and "male", each characterized by a specific combination of markers (XX chromosomes, vagina, ovaries, ova, estrogen, breasts etc. for human females, and XY chromosomes, penis, testes, sperm, androgens etc. for human males.)

Spectrum of sexual phenotypes
Recent publications e.g. in Scientific American, Nature etc. have emphasized that sexual phenotypes are diverse and do not correlate exactly with the two mating types.

While the vast majority of human beings fall neatly at one end or the other of the spectrum of phenotypes, there are also many people in whom some of the markers of biological sex point to "male" and others to "female" (and others to neither). These include people born with intersex variations, XY women with androgen insensitivity syndrome, people with chromosome configurations other than XX and XY, etc. The frequency of live births that deviate from the two binary categories may be as high as 1.7% or even 2%.

Transgender people
In addition to those described above, transgender people who are medically transitioning also do not lie at one end or other of the spectrum, because the medical procedures involved (hormone therapy, surgery, etc.) will alter some of the biological markers defining the person's sexual phenotype, so that they no longer all line up as "male" or "female". Therefore, a transgender person's "biological sex" (in the sense of sexual phenotype) may well have shifted along the spectrum away from their sex assigned at birth.

The practice of referring to transgender people as their birth sex is not simply politically incorrect; it is factually incorrect, and this terminology is increasingly being rejected by medical professionals. Since the person's sexual phenotype, rather than the mating type label, is what is medically relevant, a doctor who wants to know the biological sex of a transgender patient should want all the relevant information, not just a traditional binary sex label, the same as would be the case with a patient who is intersex.

Sex binarism and bigotry
Sex binarism refers to the belief in, and attempts to re-enforce, a mutually exclusive and collectively exhaustive dichotomy of "male" and "female" for everyone. This attitude is at the root of a large amount of real-world harm. One example is the fact that medical operations are commonly performed on intersex individuals, including children who cannot consent, for cosmetic purposes; peak bodies such as the United Nations and the Council of Europe are calling for this practice to be abolished.

Sex binarism contributes to anti-trans bigotry, with TERFs and other transphobes often attempting to privilege the notion of sex assigned at birth as a person's "true" sex. The idea of "true biological sex" makes no sense when biological sex is understood to mean sexual phenotype, since this is a spectrum and can change dramatically over the course of a person's life.

Often it is argued by TERFs that gametes (sperm and egg cells) define biological sex which is false. They often use this line of reasoning to deny the fact there are more than two sexes.