Richard A. Friedman, in The New York Times, discusses how a little sadness after sex is nothing strange, but deep unhappiness indicates something more; and that the most important sexual organ of humans is actually the brain.
Little is known about what happens in the brain during sex. In 2005, Dr. Gert Holstege at the University of Groningen in the Netherlands used positron emission tomography to scan the brains of men and women during orgasms. He discovered, among other changes, a sharp decrease in activity in the amygdala, the brain region involved in processing fearful stimuli. Aside from causing pleasure, sex clearly lowers fear and anxiety. Link to continue reading
Source: The New York Times