Essentially Markus is already a deviant early on and starts asking to pose for Carl for his paintings. He strips down for the process then gets off on the attention Carl gives him and touches himself while Carl tells him how gorgeous he is

Optional: Markus straddles Carl with perfect balance (so as to not put too much weight on him) and Carl fingers him with his long-fingered artist's hands

Anything goes as long as both parties are enjoying it! Whether Markus has genitals (robopenis and robopuss are both fine) or has a built in hole or the whole thing is wireplay, as long as Markus gets off on it and is able to touch himself. And as long as Carl can touch him and make him come :) Daddy kink also welcome but not required