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People are pointing out is that at conception there is only one cell, the fertilized egg / zygote. So everyone would have only the "large reproductive cell" (the egg). It's honestly so poorly written that you could also argue it's saying everyone's bigender too

But as an aside, it's actually not even true that XY guarantees someone will be assigned male at birth.People with Swyer syndrome present AFAB or intersex while having XY chromosomes

The opposite is true for people with de la Chapelle syndrome where they have XX chromosomes and present AMAB / intersex.

Many people with either of those never know they have XY or XX . There's also way more possibilities than just XX and XY. You can have XXY, XXYY, just X, and more

Edge cases exist, some people are xxx, xxy, xyy etc.

General concensus is that if there's a Y in the mix you're biologically male at birth but it's a bit of a grey area

Not the case at all. Remember, biology is way more complicated than you think. As a starter, there is chromosomal and hormonal sex. Chromosomal sex (X and Y chromosomes) generally determines hormonal sex. But… the gene that determines hormonal sex (SRY) can sometimes get knocked off the Y and onto the X. Or off the Y and onto nothing. Or a bunch of other things. And hormonal sex can get disrupted in a bunch of ways, so that you end up with people who are intersex or seem to be cis female until puberty when they grow a penis. Or who appear to be cis female until they have problems conceiving and find out they are XY but don’t have testosterone receptors. Or any of a thousand other things that make sex determination complicated.

Biology is statistical not deterministic and with 7 billion people there are going to be a lot of edge cases.

Initial development trends toward "female" characteristics. If an embryo has XY chromosomes then normally a gene on the Y chromosome will start to express which pivots development toward "male" characteristics.

If there's a mutation on the responsible part of the Y chromosome then that expression may not happen stunting the development of "male" characteristics.

Certain disorders can prevent the development of sex organs entirely which can lead to the testes failing to develop and subsequent hormone releases to not happen stunting the development of "male" characteristics.

Other disorders can result in the initial sex organ development, but prevent adequate hormone production which stunts the development of "male" characteristics.

Since "male" characteristics occur later in the development cycle (and not always when they're otherwise "supposed" to) at the point of conception all zygotes are of "the sex that produced the large reproductive cell" and since the current administration views zygotes as fully developed people the current administration accidentally classified all humans as female.

Incidentally there being multiple methods by which one's sex can be determined incongruously from the typical indication of one's chromosomes is just one reason why anti-trans bigotry is stupid.

Please enlighten me as well. I'm confused.