This stirs up a lot of weird emotions.

Riverworld falls into the surprisingly large group of SF novels that could accurately be tagged, “really really long, involved SF series that were confusingly sexually explicit for the age I was when I read them, i.e. 10-12.” Other examples being the World of Tiers books (also by Philip Jose Farmer) and John Varley’s Gaea series. I don’t know if there is an appropriate age for a really thoughtful extended meditation on the mechanics of a dude doing a centaur, where the centaur has both dude and lady parts. But if there is it isn’t 12.