Again, that is the biological definition which is encoded in our DNA.
But trans gender people don't claim to be able to alter their genetic makeup and so are not claiming to change sex in that sense.
Okay, let's just stop for a moment. Let's drop the bits that are "What Robin thinks ought to be the argument" and give some consideration to what is.
You keep stating your personal "ought" as if it were an "is". That's not demonstrated, and I don't accept your premise.
We can start with this little bit right here.
First, you are positing that sex is equivalent to DNA. This is your first faulty assumption. You make this assumption when you claim that the biological definition of sex is encoded in our DNA. That is false.
The definition of sex is based on sexually dimorphic reproductive phenotypes that have evolved for the production of either small gametes or large gametes. This definition is species agnostic, it holds for all anisogamous species. More relevantly, it holds for every single mammal and every single bird on our planet at present, and well back in history - back to about the time that fungi and animalia separated. This is important because
not all anisogamouos sexually reproducing species use DNA as a determination mechanism. Alligators use temperature. If the average temperature of the eggs is above a certain point, all of the offspring in that clutch will be male; if it's lower, they're all female. In humans, the primary determination mechanism is sex-differentiated chromosomal DNA. The same is true for birds, by the way, but where mammals have evolved so that the male has the mismatched pairing and females have the matched pairing, birds are the opposite. This definition of sex is also important because it addresses disorders of sexual development - it is agnostic of karyotype. While the vast majority of humans are bog standard XX or XY, other karyotype combinations exist, such as XXXY or even X0. They're rare, and they bring a lot of medical complications to the table... but they don't result in there being anything other than male or female humans.
So then, your very first assumption is flawed. Sex is not defined by our DNA, it's defined based on whether we develop the phenotype associated with the production of ova, or the one associated with the production of sperm.
Your second flaw is where the "ought/is" swap takes place. You declaratively say that transgender people don't claim to be able to alter their genetic make-up.
This claim springboards from your incorrect assumptions regarding sex. Secondly, it assumes that all transgender people AGREE that XX = Female and XY = Male. And this is not true.
Many transgender people make the (flawed) argument that sex is not based on the reproductive phenotype, nor on their karyotype... but is instead based on some combination of secondary features, or even based on behavior and social roles. The reality is that when transgender people make arguments about sex, they are very often not using a scientific definition of sex at all. They're using a cobbled together set of sex-related elements in order to support their preferred outcome. They are using flawed arguments.
You are effectively making a declaration that transgender people OUGHT NOT to claim that they can change their genetic makeup, because in your opinion, genetic makeup OUGHT TO BE considered the defining element of sex.
But neither of those things is true.
When we in this thread talk about the meme of "sex isn't real", we're talking about the arguments made by either transgender people themselves or by activists on their behalf, in which sex has been redefined to be something that it isn't... and from which they then argue that sex is either arbitrary (being a spectrum and any person can be any degree of male or female) or is a social construct.
