OH OH OH!
And that brings me to a serious pet peeve about some sci-fi novels (and really, ANY fiction) -- gratutitous and GRAPHIC sex scenes that have absolutely no bearing on the story.
It's disconcerting at best to be wading through something filled with detailed explanations of mathematics and alchemy and astronomy, and suddenly stumble into a relatively graphic scene involving someone's hand up another's behind (Guilty much, "Quicksilver"?). EXCUSE ME?
I think it's a "suspension of disbelief" thing for me. You're cruising along in your book, secure in how the story is going, your disbelief is appropriately suspended to enjoy the story unfolding before you, only to turn a corner and be smacked upside the head with a force relative to having the Mythbusters team dropping Buster out of an airplane from 2000 feet. Terminal velocity, and it's the sudden STOP at the end of the fall that kills you. WHAM. Suddenly, the spell of the book is broken.
Most of the time, I just roll my eyes and skip through the book until the scene is over and try to ease back into the story. It just didn't happen with Quicksilver. After that point, I had trouble enjoying it. I had trouble reading and paying attention, because little things kept creeping on back and throwing the whole pacing off yet again.