SpookyFrank
A cheap source of teeth for aquarium gravel
Your DNA determines your biological sex: the kind of skeleton you have, the type of germ cells you produce, and of course your genitals and secondary sexual characteristics. That's the material basis of biological sexual identity (not that the developmental process always proceeds as it should ~ developmental mishaps can occur and people with misformed or even dual sets of genitals are sometimes born).
Even at the genetic level though, sex is still non-binary. There are four or five viable heterosomal karyotypes observed in humans, even discounting chimerism which may be more common in humans than previously thought.