In possibly the greatest superhero novel of all time, Grossman follows a cyborg named Fatale as she becomes the rookie member of an Avengers-like supergroup. At one point she spars with basically that universe's Batman, a physical contest that evolves into a hardcore make-out session. While it doesn't end up with a full-on sex scene, the way Fatale describes being touched after being so radically altered by machinery is both sweet and extremely hot.
Do me a favor. If you like Game of Thrones, please go pick up Rohan's Winter of the Worlds. It's equal if not better. Following a magesmith named Elof as he wages war against an oncoming eternal winter and the evil gods that want it, he eventually travels to an ancient and corrupt kingdom. Despite being crippled while there, he still manages to get into the pants of one of the ice goddesses, and even the daughter of the king that maimed him. The latter is a sexual triumph over physical adversity that is as empowering as it is erotic.
Much of Voiez's brilliant gothic novel is a brutal exploration of the nature of rape, but among the criminal sexualities that dominate are some scenes that are cruelty-free and hot as hell. When Satori and his ex-girlfriend's best friend Raven decide to hook up, the result is a smoking-hot scene with just a hint of master and slave thrown in for good measure.
The Earth's Children books about Ice Age man are just filled to the top with engorged manhoods and slick passages. They're all wonderfully written smut scenes, but the greatest of the series all happen in the fourth book, arguably the last good one. In that book, our heroine, Ayla, has overcome a lot of the servile tendencies instilled in her from her Neanderthal upbringing and become more fully a sexual equal. Gun to my head, her and Jondalar getting it on in a hot spring is tops.