When Spider-Man rubs his wrist using his hand until a white sticky substance is released, often called, "His Webbing". Spidey often uses this "Webbing" as a form of transportation among other things in order to save the day. This is why when Spider-Man isn't horny, the day does not get saved.
Person - "Help, I'm being robbed!"
Spider-Man - "*Frantically rubs his wrist and moans* Come on you son of a bitch!"
Person - "Is he jacking off?"