Why is it ok for girls to play with boy toys (cars, ect) but once a boy plays with girl toys it's…

Why is it ok for girls to play with boy toys (cars, ect) but once a boy plays with girl toys it's the end of the world!?! If my son wants a doll house or the princess dress up clothes I'll let him. I don't get why it's such a big deal. They are little, let them explore! (And I don't really agree with transgender but I also don't agree with not allowing your child a toy just because it doesn't fit their gender)