Not gonna lie, it's a dynamic that appeals to me =3 You're right that Sayaka isn't in a great frame of mind for it, though. She's too prone to seeing it as a real obligation, rather than a game with planned boundaries that she and Madoka could slip into once in a while because it's entertaining.

We know in canon that soul gems are sensitive; Sayaka feels pain when the amoral Kyuubei presses down hard on hers. It seems logical that she could also feel pleasure if it got a gentler touch from someone who loves her. Nothing plot-pivotal for this specific fic, just a bit of my general headcanon.

Won't spoil any of the particulars, but I hope you'll like the next chapter :D

And thanks!