ADDED: Saliva is underrated. From the chapter about spit in Mary Roach's "Gulp":
We are large, mobile vessels of the very substances we find most repulsive. Provided they stay within the boundaries of the self, we feel no disgust. They’re part of the whole, the thing we cherish most.
[University of Pennsylvania psychologist] Paul Rozin has given a lot of thought to what he calls the psychological microanatomy of the mouth: Where, precisely, is the boundary between self and nonself? If you stick your tongue out of your mouth while eating and then withdraw it, does the ensalivated food now disgust you? It does not. The border of the self extends the distance of the tongue’s reach. The lips too are considered an extension of the mouth’s interior, and thus are part of the self. Though culture shifts the boundaries. Among religious Brahmin Indians, writes Edward Harper, even the saliva on one’s own lips is considered “extremely defiling,” to the extent that if one “inadvertently touches his fingers to his lips, he should bathe or at the least change his clothes.”
The boundaries of the self are routinely extended to include the bodily substances of those we love. I’m going to let Rozin say this: “Saliva and vaginal secretions or semen can achieve positive value among lovers, and some parents do not find their young children’s body products disgusting.”