An Alien Heat by Michael Moorcock
Michael Moorcock is probably best known for his Elric stuff. But I first encountered him through An Alien Heat, which was my first experience with over-the-top imagery and sexuality in SF. I still remember it vividly, all these decades later.
An Alien Heat is the first volume in Moorcock’s Dancers at the End of Time series, in which the universe has slid into almost complete entropy. Humans have become immortal and self-indulgent, using power rings to shape and reshape reality to suit their whims, living only for sexual pleasure and new experiences. So when main character Jherek Carnelian meets Mrs. Amelia Underwood, a time traveler from the late 19th century, he resolves to do something unheard of: fall in love with her and explore the strange concepts of virtue and morality.