It's more of a sense of perspective and opinion. The main problem is the fact that they are depicted as underage. The thing is, though, the nurse outfit is a sexual symbol among many others, such as maid outfits. Another thing is the hip to waist ratio of the three ponies; it is most clearly seen in Applebloom's case. Also it's become a trend that ponies in socks and other outfits is sexual. Just do what you want... 'cause a pirate is free.