I think on lowest difficulty, you get auto cleaned when you wake up? not sure. [or you just never get dirty, I never tried it]
I can't be sure either. The auto clean
on lowest difficulty might be so incredibly fast, quite instantaneous, that it leaves absolutely no memory trackers and just lets the player imagine it has happened
during the sleep. Since I have found no way to constantly check the cleanliness of bed linen in game, I am left quite clueless about the actual mechanism.
In normal or grimdark level my MC wakes up dirty - you are absolutely correct there, I think - and even then the slave girls appear oblivious of the filth -
or they prefer to let it pass w/o any fuss. At least they do not complain nor lose a single extra point more compared with the precisely same situation with other state of difficulty. At least in my recent game-effort they are blind and without the sense of smell concerning my MC:s filth
during moblow. More miracles, perhaps.