Did you get to Marin's room yet?
At the "Torture Rack" a door and stairs that lead down is where Fawn, is.
As for why Fawn, is not offering, it might be that you already accepted and like I said go back to the 'Torture Rack' go Downstairs then find Fawn.
Marin, will only talk to you comfortably if you have gone to her room and Apologized, if you have done this try to get Payback against Pixie's associate, tall Pale lady, uses a scythe, then try again.
Or You already did speak with Marin, but you didn't wear the WHOLE black underwear yet, consists of Leggings, Bra, and Panties, if you have worn them Marin, will speak with you about your apology and her first Scene should happen.
The Black underwear are named.
Black Bra?
Black Leggings?
Black Panties?
Wear all of them then speak to Marin.