I know, right?
I'm thinking specifically of one of his short stories, and I can't recall the name, but it involves aliens who want to get genetic samples from humans, so, they send an agent in the form of an attractive human female. But, due to some kind of technical difficulties, she came out of the time-space-o-mat portal only 12" or so tall, the size of a Barbie doll.
This did not keep our human viewpoint character from sticking his dick into her ridiculously spacious cooter--like the Tardis, she is bigger on the inside than the outside--the first chance he got. And he didn't know that using her as a cocksleeve wouldn't kill her.
Between this, "Up Schist Creek," the Cluster novels, and the creepy overtones in the Xanth stories ("teen audience," my ass) I am mindboggled that this isn't the general image of him among the fandom.
This is not to say he's the weirdest one out there. Read some Jack Chalker some time, like the "Well of Souls" books. They have a little bit of science fiction frosting on top but they're a guided tour of his fantasies and fetishes, full of blood, shit, and spooge all the way through. He genuinely shocked me, multiple times, and I thought I was jaded.
And then there is Philip Jose Farmer, whose Tarzan pastiche is not for the faint of heart. He decided while writing them that given that Tarzan was raised by African apes, it would be only natural that he spent his adolescence experimenting sexually with gorillas and chimpanzees. Yes, really.
I like some science fiction but damn if some of it isn't seriously fucked up, product of disturbed minds.