This is called precum or pre-ejaculate. It is useful for sex as this biologically triggered discharge acts as nature's own 'lubricant'. Watering of mouth when appetizing food is noticed by human being is an analogy to help you understand how natural these secretions of body are. The discharge you described is sign of good sexual appetite. Such lubrication is universal and happens in both male and females. This physiology is in place for thousands of years.