Hello Sheldon Cooper.......sorry Dr. Sheldon Cooper
because we are not vulcans but emotional, hormonal people craving intimacy, companionship and the touch of another human being...?
Biological imperative. It's our genetic reason for existing. Without it we die out. Although I'm pretty sure coitus is just the physical act of sex. Pon farr is the urge Vulcans have to mate every seven years. Humans are a little differrent.