Sex is a combination of genetic, hormonal, anatomical, skeletal, dermal, muscular, trichoic, and arguably neurological factors. Most of those things are to some degree malleable, and in any case their grouping under the construct “sex” is social. Sex IS a social construct. It was not written on the bones of the universe in permanent marker that there would be males and females. Sex is a correlation that humans noticed, connected, and gave a name to.

Perhaps you are instead thinking of genotype. Which yes, is largely immutable barring retroviruses and new technologies, but trans people have conclusively proven that genotype is not the same as phenotype.

By the way, biohackers have already started using CRISPR on themselves for fun.