I think you're confusing ought with is. Sex and gender are linked because gender is a hierarchy based on sex. A woman can only be an adult human female but there is also a gender aspect to being a woman under patriarchy. Society should be such that being born into a male or female body places no gendered expectations or socialization on the person but that is what we have to fight for.

Theoretically, it may be possible to raise a woman socialized male or a man socialized female if it was like on a desert island with no other contacts but in an actual society, I think it is virtually impossible. because they will get those messages from many places other than their parents. There are interesting discussions about people like Jazz Jennings or that other one that was transitioned really young and the extent to which they would be male or female socialized though but luckily this is not the experience of most trans people.