But why? Your sex is nothing more than what kind of genitals/sex chromosomes you have. It's biological. Why turn it into more than what it is? Only people who think that being one is better than the other have an issue with being labeled as male/female.
They could just raise their child to understand that -- boy or girl -- they can dress in whatever they want, play with any toys they want, wear their hair however they want, etc. It is better to teach kids that being female doesn't mean you wear pink, play with Barbies, and can only be a nurse while being a boy means wearing blue, playing with toy cars, and you get to be the doctor. They clearly conflate sex and gender roles if they feel they have to keep it a mystery so the kid can decide which box to check and which stereotypes to embrace later on. That's not revolutionary. It's nothing more than having a very late gender reveal party.