For all those other shows in Illuvatars list, the creators could well have said hetero-normative sensibility is built into the DNA and of the story. And they would have been correct.
In the old children's stories they were actually like old fashioned horror stories people told their children a 1,000 + years ago to teach children moral lessons like don't trust strangers or don't walk through the forest at night or be kind to those who you see as beneath you because when you are unkind it's from a dark heart or don't tell lies or people won't believe you when it's actually important .
The hetero normative nature of all the old childrens tales wasn't the object of the story nor the moral lesson, it was simply relative to the nature of all humans, that of love, and historically love has always been expressed between those of the opposite sex.
In the 1990:s, it was estimated anywhere between 3% and 9% of the population in the US had at least occasional homosexual or lesbian encounters (of the marital, non-marital kind).
A Gallup poll conducted in 2012 showed 5% of respondents saying the same in the anonymous exit poll, which was the higher number so overall the map was fairly unchanged in estimates on percentage of the population who would identify themselves as homosexual.
The 90's and 2000's have been very accepting decades overall, so even in a fairly open society there just isn't very many people who are not hetero normative by default.
Therefore, hetero normative interactions are what is actually normal to see historically, and thereby became baked into all the old childrens stories, especially considering all children had mothers and father's in order to be born, so it's very relatable even at a young age.
Please remember, things such as the story of Cinderella is just an adaptation of the story of Rhodopis which is 2,000 years old. These stories are nearly as old as time in some cases.
And they remain popular because they are human stories, with dreams of rising above ones station in life for those born in slavery, or morals like stay safe and don't lie and etc etc.
Children's stories don't have you going to the butcher to get castrated as a moral good and I don't believe we should start the trend of telling such horrors to the children in our society now.
What's positive is telling kids things that are good, not choose your gender for the financial benefits of atheistic butchers who want to get rich off the backs of kids.