This story has been covered pretty widely but I don't think it's been mentioned on here.
A couple have a 17 month old who they refer to as 'they' and the couple didn't even tell family what sex the child is.
Fine, keep things gender neutral and avoid stereotypes but referring to your child only as 'they' is totally othering. They will soon hear other children being referred to differently.
Pretty soon that child will be talking. Unless it lives in a bubble and never has stories read to it that child will understand there are boys and girls and wonder which they are. They will start to realise the difference between when mummy and daddy go to the toilet, unless they are never to be seen naked. At that point they will know whether they are a boy or a girl if their parents are honest and explain the physical differences between male and female.
Their parents might be able to shield them from gender stereotypes to some extent bit they can't deny the obvious biology.
Or are they going to say only some men have penises and some women do too?