Why are women more difficult to turn on sexually?

The Sexual Excitation System (SES) is the system that notices sexually relevant stimuli in the environment (an attractive person, an erotic touch, a sexy smell, etc.) and turns on. The Sexual Inhibition System (SIS) is the system that responds to all kinds of threats and turns off. This is the shutoff switch responsible for the male refractory period, among other things. If you’re in the middle of a passionate interlude with your partner, and your brother walks in the room, it’s SIS that slams on the brakes. Overall, men are more sensitive to erotic stimuli than are women; that is, they have higher SES than women. And overall, women are more sensitive to threats that reduce sexual responsiveness; they have higher SIS than men.