No, of course it's for girls. And Transformers is for boys. Our point is that it's okay for a boy to like things that are for girls, or for a girl to like things that are for boys.
Notice that the second part of that sentence never seems to be a problem; people don't tend to have an issue with a girl liking Transformers. This is because boys are seen as better than girls. For a boy, playing with a girl's toy is seen to be demeaning, because he should be playing with his own (better) toys.
It's an ugly, outmoded, sexist way of thinking. Girls are just as good as boys; women are just as good as men. Girls can play with trucks if they want to, and boys can play with dolls if they like, and neither of those things should be weird.
(Note that this has no bearing on adult creeps sexualizing children's shows, or neglecting real-life relationships in favor of fantasy ones, or surrendering their identity to a cargo cult of ponies. A brony can still be creepy for a wide variety of potential reasons. His gender should not be one of them, though.)
I apologize for the derail, but I really dislike sexism, and it's one of the things the thread has (rightly) called bronies out on. Let's not go down the same path.
Boys are universally recognized as better than girls. by who? oh no, not me, heh. here let me tell you about my cartoon pony collection