Superstimuli for sexiness are like superstimuli for taste: take the reproductively best things in the ancestral environment, and exaggerate their characteristics outside of what was found there. For example, prominent sexual characteristics are sexy; Escher girls made mostly of breasts and buttocks are therefore sexier. Watanuki is such an example; the gangliness and androgyny are are but possible, the legs and facial structure are exaggerated but not that far from realistic equivalents.

Also, are many people sincerely attracted to Jessica Rabbit? It seems to me like she only represents sexiness, like we immediately understand a stick figure in a skirt to mean "woman".