His response was completely justified given his observation of his surroundings & media such as YouTube & TV. The under-representation of women in this context formed the bias that girls can’t do this. Now, for kids that age, superheroes represent strength, freedom, & the interminable ability to save the world. For that kid, girls now represent being the sidekick & being weak. He might outgrow it, but this bias formed in his mind without any active effort.