Arousal: Arousal slowly raises over time. This can be sped up by watching any Kinky TV channel, and (I think) seeing other sims naked or woohooing. When a sim gets super-horny, they get the "Horny Curse". To relieve excess arousal, your sim can masturbate for partial relief, or have genital sex. (Teasing or oral sex makes sims even hornier.)
Exhibition: not sure if it's a motive, there are no moodlets or bars for it. Sims with the Exhibitionist trait do seem to flash their bodies a lot though.