I think the underlying message is: in a patriarchally structured society, it's ok for girls to dress 'a bit' like boys because "of course some girls want to be like boys, everyone knows men are better/more important than women."
But if, heaven forfend! - a boy wants to dress like a girl or a man wants to dress like a woman* - then this is seen as weakening or demeaning to not only that individual but to all penis-havers everwhere - "because everyone knows men are better/more important than women, and so a man who is too like a woman is somehow letting the side down", or some such nonsense.
It's adorable, or at least only a bit threatening, for a girl to be a 'tomboy,' to try to be 'as good as a boy'. It's downright panic-inducing if a boy VOLUNTARILY gives up some of his male privilege to dress as the 'lesser' sex, even for a temporary period of time - because OMG it might reveal that all male privilege is basically bunk, someone will notice that the Emperor is not wearing any clothes, and maybe the whole system will come tumbling down! Quick, get that boy into camo gear, pronto!
*obviously, by 'like a woman' I mean in stereotypically feminine clothes. And not like this woman writing here, who is wearing jeans and a shirt. Like a man. OR a woman.