Sex is any process that mixes genes from different individuals.

People tend to harbor essentialist beliefs about sex — that is, they presume that each sex category has an underlying "essence" that makes them what they are. This is what leads people to assume that trans women remain "biologically male" despite the fact that many of our sex characteristics are now female. However, there is no “essence” underlying sex; it is simply a collection of sexually dimorphic traits. Some people will presume that sex chromosomes must be this "essence," even though we cannot readily see them, plus there are non-XX or XY variants. Others presume that genitals are this "essence" (probably because they are used to determine our birth-assigned and legal sex), although they can vary too, and may eventually change (e.g., if one undergoes sex reassignment surgery). In day-to-day life, we primarily rely on secondary sex characteristics to determine (or more precisely, presume) what sex a person is — and of course, these traits may change via a simple hormone prescription. Like I said, there is no mystical “essence” underlying sex.

The way I see it is this: everyone has a biological sex, and for most of us it’s unambiguously male or female.

sex is merely a genetic joint venture. the process of choosing somebody to have sex with, which used to be known as falling in love, is mysterious, cerebral, and highly selective.

Sex is recombination plus outcrossing; their mixing of genes is its principal feature….So sex equals genetic mixing.

We were born boys or girls, but we don’t know what we will become, to which gender we will belong to at death.

Women and men are divided by gender, made into the sexes as we know them, by the social requirements of heterosexuality, which institutionalizes male sexual dominance and female sexual submission.

Sex is a collection of traits that, while generally dimorphic, can vary greatly in the population, and some can change over time. While the terms "male" and "female" have some utility, we should not view them as strictly dichotomous or mutually exclusive. Rather, “female” and “male” are best thought of as umbrella terms that describe groupings of people (or animals) who generally share many of the same traits, albeit with considerable variability and some exceptions.

Held up as a proof of socially constructed gender roles, he proved the exact opposite: that nature does play a role in gender. The evidence from zoology has always pointed that way: male behaviour is systemically different from female behaviour in most species and the difference has an innate component. The brain is an organ with innate gender. The evidence from the genome, from imprinted genes and genes for sex-linked behaviours, now points to the same conclusion.

Proof that the penis is a much later development than the female vulva is found in the evidence that the male himself was a late mutation from an original female creature. For man is but an imperfect female. Geneticists and physiologists tell us that the Y chromosome that produces males is a deformed and broken X chromosome — the female chromosome. All women have two X chromosomes, while the male has one X derived from his mother and one Y from his father. It seems very logical that this small and twisted Y chromosome is a genetic error — an accident of nature, and that originally there was only one sex — the female.

Why has that man fallen in love with that woman? Because she’s pretty. Why does pretty matter? Because human beings are a mainly monogamous species and so males are choosy about their mates (as male chimpanzees are not); prettiness is an indication of youth and health, which are indications of fertility. Why does that man care about fertility in his mate? Because if he did not, his genes would be eclipsed by those of men who did. Why does he care about that? He does not, but his genes act as if they do. Those who choose infertile mates leave no descendants. Therefore, everybody is descended from men who preferred fertile women, and every person inherits from those ancestors the same preference. Why is that man a slave to his genes? He is not. He has free will. But you just said he’s in love because it is good for his genes. He’s free to ignore the dictates of his genes. Why do his genes want to get together with her genes anyway? Because that’s the only way they can get into the next generation; human beings have two sexes that must breed by mixing their genes. Why do human beings have two sexes? Because in mobile animals hermaphrodites are less good at doing two things at once than males and females are at each doing his or her own thing. Therefore, ancestral hermaphroditic animals were outcompeted by ancestral sexed animals.

I will say this categorically — that you cannot change your sex. Your sex actually is there in every single cell in the body; you have a chromosomal sex, you have genetic sex, you have hormonal sex, you have... psychological brain sex.

My genes have indeed determined what I am, but only in the sense that, given the succession of environments and experiences that were mine, a carrier of a different set of genes might have become unlike myself.

Sexual relations are driven not by what is good, in evolutionary terms, for men or for women, but for their chromosomes. The ability to seduce a woman was good for Y chromosomes in the past; the ability to resist seduction by a man was good for X chromosomes in the past.