You are slightly misremembering a rather obscure book called A Splendid Chaos by John Shirley.
Zero is a young film maker who believes his whole life and career are mapped out before him. That is, until the night he and his friends walk into a rock club ... and are caught in a dazzling trap that spans worlds. They are dropped onto a dreamlike planet whose surrealistic beauty cannot hide its grotesque reality. Fool's Hope - a world, so stunningly bizarre, nightmares are irrelevant. Here, abductees - both human and alien - are pitted against a neverending succession of hellish parasites, carnivores, shape-changers, and symbiotes.
Yet the greatest enemy of all could be human. When former professor Harmon Fiskle is transformed by the Current — a roving mutagenic force - he is freed to pursue his megalomaniacal nature. He advocates a depraved policy of social Darwinism, and forges a grotesque alliance of Twists: men and women who have sacrificed their own humanity to become monstrous mutations of their former selves.
With an entire world at stake, only Zero can solve the mystery of Fool's Hope ... if it isn't already too late.
The aliens are High Clan not Klansmen:
Looking again, Zero saw that one of the men wasn't a Pioneer, wasn't even quite human. His skin was black but not Negro black; it had the waxy blackness of licorice.
His eye, or perhaps eyes, was a strip of sparkling white-gold above a nose
that looked as if it had been split with a blunt knife and never sewn up. He
was nude except for a metal-mesh loincloth, intricate bodypaints in floral
abstractions, rather like Louis XIV wallpaper, and tennis shoes—Reebok
tennis shoes he'd bartered from someone. Four silvery hoops pierced his
right wrist the way earrings pierce an earlobe. A black plastic box on his left
shoulder translated for him as he spoke to the others. His hands seemed
entirely human, except the fingers were perhaps an inch too long.
"That one of the Twists?" Zero asked, whispering. "Looks like he might've
been human once."
Bowler shook his head and murmured, "An alien. High Clan. Their trading
rep, Zickorian. Supposedly one of our allies, but, like, no one quite trusts
him completely."
The mating ritual scene is:
"Don't you dare!" It was Angie, shrieking in outrage from the other side of
the rock.
...
She snapped, "They came up to me and started making those obscene
motions, and then Zickorian took a stick out of his satchel and started
rubbing it on my crotch!"
Zickorian replaced the stick in his pouch with one hand; with the other he
thrust a finger into a corner of his mouth, which Zero had learned expressed
puzzlement and surprise. "You mean her protestations were genuine? They
seemed to translate so exactly to the ritual protestations of the matable
female."
"She means it."
"Then why did she signal for a mating ritual? Just as we'd completed our
mollification of the animal-invested deities. That is the proper time for the
female to squat and emit musk."
