Here's an interesting story.
Captain Underpants and the peeved principal
While I think that the girls were being creative in their costumes, I think that they probably knew they would be pushing the envelope of what is acceptable.
I'm reminded of a student, during my ninth grade year, who came dressed as a giant condom, and then acted like he didn't understand while the school administration asked him to take it off.
If you are going to push boundaries you have an idea that others may be offended and upset. Nudity, vulgarity and bathroom humor, even if it's the hint of the idea, will always push boundaries.
If you push boundaries, do not come back with, "I don't understand why their upset." You have a clue. Just take responsibility.
I more turned off by the fact that others try to convince me that they are clueless, than the issue behind their cluelessness.