Know why just looking at an attractive face stimulates sexual arousal? Well, it takes two to tango!
The behaviours like seeing, smelling and sexual arousal that "come naturally and do not have to be learned" occur because of two classes of pheromone receptors that are crucial to triggering the mating process.
Pheromones are chemical signals that let an animal know when a suitable mate is near and activate the release of hormones that encourage the animal to mate.
In mammals, the vomeronasal organ, located between the roof of the mouth and the nose, detects pheromones.
Humans no longer have a functional vomeronasal system but the mechanisms that control our inborn behaviour are similar to those in other mammals.
For example, looking at an attractive member of the opposite sex can stimulate hormonal changes in humans that lead to sexual arousal.