I like the idea a lot, but making your job even harder for the sake of it is a big no. That's how burnout happens and great games get abandoned midways. Plus having to micromanaga all of that would be quite a pain.
... Perhaps if it becomes a job, like hunting or forging. Make it similar to prostitution, but focused inwards for a blanket efficiency bonus instead of coin. Say, something like a Service job, with options of "attention" - or whatever you wnt to call it - that relieves stress and/or gives a 'well-rested' buff to people resting, and a "relief" option for the sexual version. It could work with the current interface, much like picking either leather or meat when you assignt hunters to their job. That way, you get the benefits without having to manually make slaves have sex regularly. You could add a new upgrade room , like a resting room, a linen closet or something of the sort or let it be implied they do the attending and the "servicing" on the slave/employee's own room. You could even make use/give another function to the Maids/Butler job that way: they can make the resting action/room all the more efficient at removing build-up stress when assigned to "attention", while Harlots get a bonus to sex relief and Geishas can do pretty well in either,
Would that help? Is it more doable like this? It makes use of mechanics already there, so hopefully that makes this wonderful idea easier on you.
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Another idea I had while writing this is the possibility of a "bar" or perhaps a "restaurant" upgrade. Once built, it could unlock a job option as a waiter/waitress - a job in which Maids/Butlers get a bonus in, as their training makes them betters hosts and thus more likely to get tips (i.e. extra money to your pocket, even if only a cut of it for non-slaves) - and have them serve food and drinks for a passive income that does not need you going to the market every time, much like Prostitution. Also, keeping the pantry full of delicious food and liquors could be both a reason to invest ever-valuable exp into the Cook class and something to keep your cooks busy with, and with well-equiped hunters, fishers and farmers, it could become a good source of passive income.
Would either of those ideas be useful to you?