Carnavale precedes a period of traditional penitence and self-denial leading up to Easter in Catholic countries. Sexual and other acting out prior to this period of Lent are given into with the anticipation of later forgiveness.
As @Dr_Lawrence and @Simone_De_Beauvoir pointed out this is a period for cutting loose and pushing social boundaries. The interesting thing is that nudity is everywhere today so it is no longer unconventional. When you can turn on the TV in mid afternoon and see Paris Hilton in a string bikini humping a car while eating a burger. a girl dancing in feathers is hardly shocking, Nudity at Carnavale is becoming a quaint little tradition.