Farmer is not afraid to write about sex and some of his material is extremely X-rated, far more perverted than anything I've read by Stephen King. Farmer was the first author to write about human/alien sex and Robert Heinlein dedicated Stranger in a Strange Land to him, for paving the way. (Reading material by Farmer will add the word satyriasis to your vocabulary.) I can see religious people banning his material, but not for the hardcore sex. They would ban it because he wrote a few stories about Jesus being an alien, and not the son of God. One novel in particular, Flesh, is similar to Planet of the Apes (the screenplay written by Rod Serling), except that the astronauts return to Earth and find a new religion, rather than a new dominant species. One Christian man is killed when he attempts proselytization of some locals because Christianity is no longer the dominant religion. This indicates to me that religions are transient and not the sources of absolute truth many believe. In other words, God may go out of style. This is the advantage of reading Farmer. He will expand your imagination.
These are the books I have read by Philip Jose Farmer so far:
Besides Riverworld, the other notable series by Farmer is World of Tiers, which comprises seven novels. They are the last 7 scans above. It's about near-immortals (people that live roughly 30,000 to 50,000 years) and have the ability to make their own universes. Gates, like those in Stargate, are used to traverse the universes.