...is the brain.
Sexual stimulation starts with sensory inputs that trigger mental associations. Only then do the other parts of the body get involved.
For each person, the sensory inputs that trigger this reaction are different. Seeing an attractive person, feeling loved, hearing a whispered word, experiencing a certain kind of touch, all can set off the human libido.
Sex and intimacy are intertwined -- many people have trouble feeling sexually interested in someone until they have had time to know one another, learning to trust through words, actions, and nonverbal communication such as facial expressions and body language.