Responses
It might be a "curse" instead of just the illness the previous person have
From what I can remember, the top prostitutes in that place were force fed special aphrodisiacs that they made them addicted to the drug and sex??? This is BL logic btw, not how actual drugs or sex addiction works lol. So it's not that the stepfather's body had a natural high sex drive but he was addicted. The dragon doesn't want to fuck (which is essentially the drug) so he's experiencing withdrawal.
But why is demon/uke's body in so much pain because of horniness??? It doesn't make sense? :3 I understand that the human body was of prostitute so I could understand high sex drive or something but how come it's making the demon so sick lol clearly this demon has capacity to entirely hide away the human soul to an extent that eye color hair color everything changed