I have to admit it sounded a bit weird to me at first, but as Carlos Maza argues, there are a lot of silly pressures that society puts on kids, unwittingly, to conform to one gender or another.
By refusing to disclose Storm’s biological sex, then, Witterick and Strocker are protecting Storm – at least temporarily – from the incredible pressure that society tends to put on young children to conform to certain gender stereotypes from an early age: boys play with trucks and like the color blue, girls play with dolls and like the color pink, etc.And as Anderson Cooper's ongoing series on CNN attests, bad things can happen when parents freak out over their kid's gender non-conformity.
By allowing Storm to grow up without these kinds of external pressures, the parents are creating a space for Storm to express his/her gender identity authentically.
And here's the original story about the parents, and kid. It notes that the parents, siblings, and grandparents know the kid's gender. I wonder whether that still subconsciously influences how they treat the child, whether or not they realize it or intend it.