Beginning in 1897, Henry Havelock Ellis dedicated his efforts to an extensive exploration of human sexuality, captured in a six-volume series known as Studies in the Psychology of Sex. His work is notable for its candid case studies that often delve into unexpected and explicit themes, reflecting the complexities of sexual behavior.

Among the intriguing insights from volume four, entitled Sexual Selection in Man, Ellis highlights an unusual observation: even a simple interaction, such as a dog's tongue touching its mouth, could trigger sexual pleasure. This particular example illustrates the range of stimuli that can invoke sexual responses, further emphasizing the intricacies of human sexuality.