Dumbledore decides that anything and everything is acceptable for the Greater Good, including pimping out fresh young students (but most especially, Harry) to lustful adults in exchange for support. This should focus primarily on the Boy Who Lived and wealthy or powerful older ladies who want to get some of the famous meat. It could start at any time during his Hogwarts years, though the best times may be after Voldemort arrives or straight shota, with him in first year. Harry doesn't know what Dumbledore's doing at first or resists but eventually capitulates to the idea that this is the best way to win support for the Light. Maybe his Savior Complex comes to the fore and he reasons that this helps the most people while only effecting himself. In the end, Harry spends most nights with a older lady bouncing on his cock, with Dumbledore serving as his pimp and handler.
What I'd Like to See:
Harry and Umbridge (A Sexier version) - Dumbledore trades Harry in exchange for Ministerial support against Voldemort.
Harry and Parent of Some Classmate, where the woman fucks Harry, is impressed by how big and hard he fucks her, and teases that he'll get her daughter to take Harry on as her handsome boyfriend.
Dirty Talk, mild humiliation of the lady, maybe mommy-son kink.
