Here's a thought for dickgirls: They typically have both sets of genitalia, so depending on the fetishist, it could be a desire to transcend the current notions of sexuality, to be both Male and Female, to both give and receive "naturally."
As for ones I, personally, can't understand: Vore, Gore, Scat, Tentacle (Rape), Bodily Fluids/Functions, Feet, Loli/Shota, Flat Chests, Bestiality, Chibi/Pixie (Where the girl is big enough to fit in the palm of the guy's hand or thereabouts), Ganguro, Giantesses, Cuntboys (like I said, dickgirls can at least make some sense, their logical opposite, on the other hand), Multiple Breasts/Dicks, and Rape.