How is sex a great lie detector?
Involuntary physiological reactions such as the wetness of a vagina and the stiffness of a penis are emotionally so satisfying (which means, simultaneously, so erotic) because they signal a kind of approval that lies utterly beyond rational manipulation. Erections and lubrication simply cannot be effected by willpower and are therefore particularly true and honest indices of interest. In a world in which fake enthusiasms are rife, in which it is often hard to tell whether people really like us or whether they are being kind to us merely out of a sense of duty, the wet vagina and the stiff penis function as unambiguous agents of sincerity.
Excerpted from HOW TO THINK MORE ABOUT SEX by Alain de Botton