Reproduction is in fact a biological imperative - your gene line will die without it. People don't eat food because they know they'll die if they don't, they eat because they're hungry. Biology tells the animal what it needs to do. There is a natural drive even if it's not technically a requirement for the individual animal (human) to survive in the environment.

There is a physical aspect however - libidos don't exist purely in your head. Hormones have effects.