^I suppose the nose would make a good point of comparison - the protective outer covering is no more sensitive than any skin, while the inside is all mucous membranes and whatnot and quite sensitive indeed. ('Course the very function of gills involves constant passage of water plus whatever particulate matters now may happen be floating in it through them so high tactile sensitivity there would be pretty counterproductive and distracting...) Sticking your finger in there is not generally considered a terribly erotic experience tho, and in the configuration depicted here they'd be opening directly into her lungs which sounds all kinds of y a b a i to me...