I believe this to either be a birth defect or a serious psychological problem. If you like either of these things in a sexual way I suggest you find help somewhere. No one will like you if you tell people beyond the internet.
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Actually, I read somewhere that the foot fetish comes from basically a bad link in your brain that associates feet with the other erogenous zones due to proximity. It's also the most common. Diapers, on the other hand, are just -blam!-ing gross.