I've recently been digging back into the archives for Spuffy fic, so I have no idea how new/old this one is.

I specifically remember at one point Buffy and Spike were in bed, and he told her he loved her - in a sexy way, not a hopeless romantic way. She had an unexpected reaction - eyes rolling back, finding it pleasurable. Spike's internal monologue was similar to: 'she might not be willing to accept it, but she's not opposed to hearing about it' - and then Spike proceeded to describe how he felt and what he was doing/going to do to her. It was very NC-17.

I hope this rings a bell? Thanks!