I have a psychology degree, and I'm back in school pursuing a nursing degree, so I'm currently retaking human development. Both when I originally studied it about 12 years ago and today, we distinguished biological sex from gender and its social and psychological components.

Biological sex is absolutely real, but it isn't synonymous with gender. Biological sex concerns reproductive anatomy, chromosomes, hormones, and other biological characteristics. Gender also involves identity, social roles, and cultural expectations.

Children begin learning gender roles and stereotypes through socialization in infancy. According to gender schema theory, by around age three, most children can label their own gender. Around four, they develop gender stability, understanding that boys generally grow into men and girls into women. By around six, they develop gender constancy, understanding that changing clothing, hairstyles, or activities doesn't necessarily change someone's gender.

Interestingly, children tend to enforce gender stereotypes most rigidly around ages five to six. By seven or eight, they generally become more flexible and accepting of peers who don't conform to traditional gender expectations.

So yes, biological sex exists and is relevant to sexual reproduction. But that doesn't mean the social roles, expectations, and identities associated with sex are themselves biological facts.