A Marriage Contract Porn Fic

This is PORN, with a bit of plot. There's a futa, and it's a F/M/F/F situation. Due to a marriage law, Harry get's matched with Millicent Bulstrode. Daphne Greengrass sees her chance and manipulates Harry into a very restricting contract, making him her sex-toy. Some rituals later, and that designation is pretty much literal. At first he hates it, but soon he begins to see benefits