if you say you want it with nameless, is it a game over?
Not really, but keep your sanity full or atleast more than 25. If you'd give in to him, your sanity will simply be subtracted by 25.
But if your sanity is below that, this will result on having your sanity to 0. In which, it's a game over.
Though if you're interested what the game over is like. I suggest that you copy your lustfull desires file on c:/ users/ (name of your account) / appdata/ roaming/ renpy. Save it somewhere else before you encounter nameless.
This way if you mess up your saves. Simply delete the old file and the game. Then reinstall the game and recopy the copied file and paste it back to the directory I've stated above. I still suggest keeping the 1st copy incase you might mess up again.