The principle behind it is that it teaches kids that relationships must involve a man and a woman conforming to stereotypical gender roles, and implicitly that not fulfilling these ideals is abnormal or wrong. This is obviously a very narrow argument that ignores all other factors of a child's upbringing (e.g. parenting, environment, the very children's media they praise for subverting this), but it doesn't really matter to them, since it's just cartoon sperging under the veneer of profound social progress.