There re those, and for awhile I was one, who say biological sex doesn’t exist. Supporting this is the reality that most of your body couldn’t care one fig what your chromosomes say. The exception was the development of your ovaries or testes or possibly the ovo-testes that form in the development of some intersex people.
All else is a matter of destiny, having been determined by that switch that caused one set of reproductive organs to form over another. It is (as far as I know) only the gonads themselves that are dictated by genetics, the rest is the product of human development.
Indeed, one of the things that we as trans people say is often supported by noting that sexual differentiation in the reproductive organs and the brain don’t happen at the same time.
However, it is also reasonable to segregate babies into three piles. There are the indisputably male and indisputably female babies. These represent 98+% of all live births. Most of them will go on to be congruent cis people and a substantial minority will be LGB or T.
The other 1+% of babies will have some developmental and/or genetic issue that has caused them to have somewhat or very atypical reproductive organs with respect to their chromosomes. A tiny additional fraction will have one of the ten or more arguably abnormal sex chromosomal conformations like X, XXX, XXXX, XXY, XYY. There are labels like Turner’s syndrome and Klinefelter males that describe the results.
What does biological sex mean? It is reasonable to say that for that 98% or so of people who have the typical genes (XX or XY) and have predominantly normal sexual development that there are two biological sexes.
Because of the hormones that the genetically determined gonads produce, the females will be smaller, have wider hips and have breast development and a female reproductive tract. Similarly the males will be mostly larger, stronger, have more narrow hips, more body hair and a male reproductive tract.
For those with “normal” biological development, it is tremendously useful to assign a sex label because it is useful in everything from who should compete in sports, to how people are treated medically (a real issues for trans people) and how marketing is done to the different sexes.
Here’s the thing. Every person is a combination of a million things that can be measured, and some of those measurements equate to a person who is functionally female and some to a person who is functionally male. As above, that covers 98+ percent of everyone.
What the people who use the term “biological” sex want it to mean is only those individuals who are born of a sex and develop into that sex should be considered that sex. It is there we get into trouble. We do for the same reasons that we get into trouble if we forget that not every baby can even be classified male or female. In a good sized high-school of 2000 young adults, about a classroom (30) of them will be intersex, and that can range from having the entirely wrong set of reproductive organs compared with their chromosomes, to having one or more very minor condition as I do where I had an undescended testes that had to be resolved with surgery.
In that same high school, about another ten young adults will be trans, and of the entire student population about 60 individuals will be predominantly oriented toward same sex partners. Even with some overlap that means that 1 in 20 (about 4%) will be gay or trans. Past that there are still more who will be bi or gender non-conforming. When you get done you probably have about 10% of the student population that is not the normative cis-straight story.
What complicates this even more is that trans people have brains that are closer to their identified gender than their physical sex. That their brains fit this mold has now been shown by both gross anatomy examinations of sections of the brain that differ from males to females, as well as connectivity comparisons which, in my opinion, starts to prove that trans and cis of the identified gender actually do have similar minds.
In fact, enough proof has accumulated over the last twenty five years that it would be extremely difficult to argue that dysphoria is an imaginary problem. Like many things that started unexplained, and therefore explained away, this is another case where the real life experiences of people were born out in scientific investigation.