Scientists believe that disgust evolved as a defensive mechanism to keep us from being contaminated by external sources.1 Accordingly, the mouth and the
vagina, two body parts that lie at the border
of the body (and are therefore at a higher risk for contamination), demonstrate greater disgust sensitivity; for example, we are likely to be especially grossed
out by having a spider
crawling on / around the mouth or
vagina compared to, say, the left arm.2 Add to this the finding that some
of the strongest triggers for disgust are body odor, saliva, semen, and sweat, all heavily involved when getting «down and dirty,» and you can see how the relation between sex and disgust seems contradictory or even obstructive.