Why is it that it's perfectly fine for girls to cross-dress and not guys?

because our society and probably all societies view male and female uality as completely different. the crossdressing thing is just one of a large number of differences along these lines, e.g., the fact that men are supposed to have more partners than women, that men are usually older than women in a relationship, and others. and plz note that there have been times in history or cultures where things are/were different. joan of arc was largely persecuted and finally burned at the stake for dressing like a man, and on the other hand many societies tolerate a cl of men who dress like women and act like women.