At the end of second year have Female Dobby claim the life debt that Harry owes her from when she saves him from Mr Malfoy.
Use the trope of a house elf always needing to be bonded to some one to survive but when Dobby attempts to bind herself to Harry instead of Dobby being a normal slave Dobby because becomes a sex-slave.
Have Dobby embrace her position as Harry's sex'slave and do her best to become the best slut for her master she can be. And have the bond make Harry accept his position as Dobby's sex-master.
You can keep it as only Harry being only Dobby's master or you could add Other Elves like Winky.
