I said that if I could suspend disbelief for vampires and werewolves and the like, I could very probably temporarily suspend disbelief for anatomies that wouldn't actually work that way. For me it is the same kind of thing. I can understand why other people wouldn't be able to suspend disbelief, though. There's only so much room down there, and a certain spectrum of variations that are humanly possible.
It appears there are an amazing number of futanari stories out there, too. I'm sure there there are books that could be written on the socio-cultural implications of it all.
I would like more stories about trans* and genderqueer and intersexed characters out there, serious and thoughtful takes on it. I don't know how much of an audience there is, but I know there are writers who can do it well. There's something to realism that strikes me more.