There is another big reason for the difference. The skin between your index and middle finger are not "responsive" that is swelling/engorging with blood during sexual arousal. The breasts (nipples), the genitalia - male and female, do "respond " to stimulation and arousal. These are the places usually kept covered. It is not just social mores, but due to physiology that make an area of the body "sexual". Yes, there are dual purposes to the organs such as feeding an infant, urinating, but that doesn't change that they are also "sexually responsive".