It's not that I'm a prude or anything. I've done it myself in my twenties several times (for a party, not ahem private titillation)
It's just lately since I've had my own child, and with so much public awareness of paedophilia and its apparent standing as THE crime people hate most, it just seems to grate on me that people can think it's still ok to sexualise what is essentially a child's outfit.
I suppose what I'm trying to say (but not very well) is that it's a bit like dressing as a Nazi for a party. It's not funny because of all the bad connotations & just not the 'done' thing.( but not quite the same, IYSWIM)
I am probably BU, but it just makes me feel a bit 'eew!'