My first suggestion is, that the prostitution job is getting a risk-factor (like 5 or 10% per day) that something bad happens, something in the range from had a rough lover and got a temporary flaw like a blue eye (which decreases task-efficiency) to get real damage like 25% of the servants hit points.

Maybe also having a chance that a customer just doesn’t pay and a chance for a really bad event where the player got a small quest to get the servant back from some slave-traders, by paying, negotiation or fighting.

The second suggestion is to have a brothel upgrade that have just a few slots. Of course, is the Idea that these slots have a higher efficiency than the normal prostitution or are just safe (in case the first suggestion becomes part of the game).

In this brothel could also be a second job (where every servant also blocks a normal brothel-slot) to gather information that could be sold or used for something else but getting less or no money without selling these.