Where?
I'm sorry, if you haven't read the book then I can't quote the entire "what changelings are" section for you. A few lines, sure, but after that copyright kicks in.
Err, this states "feeding" - as I'd thought. Do you have a reference stating "sex"? Perhaps it's worth noting, the act of sex isn't what they feed on. WCVs feed on emotion.
Here's a bit more from that section:
"Killer sex," I said. "Literally."
"To die for," Bob confirmed.
An eerie thought, and one that disturbed me a lot more than I thought it should. "What if the vamp doesn't want to feed on someone?"
"Want doesn't matter," Bob said. "They feed on pure reflex. It's what they are."
...
Richard