Also I'm not accusing anyone of pushing this on their kids, rather, our society does it in massive doses. Watch most children's TV shows or movies and heteronormativity is abundant.
For a pretty clean example of this check out the movie Up by Pixar. The first 10 minutes of the movie are devoted to Carl & Ellie's relationship and the loss of that relationship serves as a major driver for the movie.
Also there's nothing wrong with showing children this kind of content. It helps them make sense of the world. However, there is an issue with representation. When it's all they see then it constrains their minds as to what is possible.