it's caused by the fact that the brain has areas designated for recognizing body parts (and other various things) - in men, the areas for recognizing feet are right next to the areas for recognizing female genitalia, and in some developmental processes of the body, these two regions "merge" and therefore make it so that while the person won't see the part as another, necessarily, they will have similar chemical responses in correlation; such as arousal.
It may just be me but I don't really mind about shape, texture or anything. Only thing that comes to my mind is ass and like 90% ass is acceptable for my liking
basically as long as da pusillanimous individual gud I'm happy