I remember Simon was very ambivalent towards Jayne- I think he enjoyed the physical aspects of sex, but not who he was with, and was frequently surprised by Jayne's gentleness in the bedroom. I think it followed both characters so you got both sides of the story, and Jayne was hurt and disgusted when he realised why Simon had suddenly taken an interest in him. I don't think it had a happy ending.
Jayne had been surprised at his luck, bedding someone as pretty as Simon, and couldn't understand why he kept coming back for more after having a bit of rough. He had started to fall for Simon, tentatively doing nice (misinterpreted) things for him and wanting to cuddle afterwards, whereas it was all Simon could do to keep a smile on his face whilst going through the motions. It was quite a heartbreaking fic tbh!
If no one knows what I mean but has read anything in a similar sort of vein, I'd love to have your recs!