Saliva: Secret Ingredient in the Best Kisses | LiveScience

"Salivaincreases sex drive.
...'There is evidence that saliva has testosterone in it,... and testosterone increases sex drive. 'And there is evidence that men like sloppier kisses with more open mouth. That suggests they are unconsciously trying to transfer testosterone to stimulate sex drive in women.'

Men also could be using the saliva transfer to assess women's fertility and estrogen."