Nudity - Reactions
Unless disabled in settings, Sims will react to seeing nudity. The default in-game reaction to nudity has been completely replaced by the WickedWhims reactions system.
Sims will always attempt to have a full reaction with an animation when first seeing a nude Sim. Every next reaction to that same Sim will be done with an inner reaction, without an animation, but instead represented by a thought bubble.
Reacting to nudity depends on the situation. Nude Sims or Sims that are used to nudity (like having high Naturism or Exhibitionism skill level) will rarely react to seeing other nude Sims.
Reaction to nudity can be either positive or negative. The outcome depends on the age, traits, relationship between Sims, and a little bit of randomness.
No matter the reaction type, every Sim, especially the NPC Sims, will gain skill influence that will help them progress the Naturism or Exhibitionism skill faster.