> something in the individual's brain has realized that what it's getting from the eyes is supposed to be exciting.

If you motion captured someone dancing sexually I would recognize what it's supposed to be even if you applied that animation to a wireframe of bones. I could even do that even if I didn't find grown women (or men) dancing sexually attractive myself, just by osmosis, just like I know an ad for a car is supposed to elicit this or that association, while not caring much for cars myself.

> The self-disgust this causes is

an assumption of yours, and you might as well ask what makes you engage in these mental gymnastics. I don't find children dancing sexually arousing in the least, yet still recognize it as dancing sexually, which isn't weird or interesting at all, much less "ugly".