Please consider the amount of potential work which would have to be done before making a suggestion. Adding sex schedule is a whole new area of complexity which i simply dont have both time and willingness to do.
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A simple schedule, as in an option on every slave to automatically have sex whenever their lust is too high, with additional options to pick either the highest lust slaves as partner, their favorite slave, or let the master decide. You can decide to punish slaves by denying them sex, or make them serve others. Slaves assigned to the Service job will join any open sex slot to increase the enjoyment of other slaves; they will not enjoy the sex itself, but some classes decrease the tiredness gained from the Service job.