Sex - Reactions
Unless disabled in settings, Sims will react to seeing sex with with one of the 16 available positive, negative, and jealousy outcomes.
Reacting to sex depends on the situation, mood, traits, age, clubs, activity, but most importantly, relations between Sims. Encountering cheating will result in jealousy, but otherwise, Sims can get horrified, angry, horrified, sad, embarrassed, playful, excited, flirty or just stay neutral.
Sims with the Cuckold/Cuckquean trait will not feel jealousy from watching their partner in sex.
Sims in sex clubs will react generally positive to seeing sex of other members.