The easy way

"What do you need my body for? What will we do?'' you ask, in an attempt to show some manner of obedience. ''It would be easier if you removed some of the... features... you gave me. I would have an easier time helping you with a body closer to what I am used to."

"You do not need to worry about any of that. You just need to start moving." Even as it talks, the voice is starting to get quieter, almost... weaker?

"I need to rest now. Communicating with this body... molding it... has taken a great deal of my power. Continue onwards, and when I feel you need further... guidance... I will return. Disobey me at your own risk." You feel almost as if something has left you, a weight lifted. It didn't feel like being alone before, but now it does, as the demon's presence retreats elsewhere. The sense of artificial arousal leaves your body, your new cock slowly deflating back to a flaccid state between your legs.