I think he did it to manipulate MC on getting him to speak about the phone ? Or trying to get the phone from his clothes. Since it slipped out he don't need to go and do "it" further.
Soo I think he'll manipulate MC from here on because of the phone incident with the detective, he'll do anything just to make him talk, and I think that's what he meant by his "traps" for him in chapter 21
Hsh?? Im confused? Whyd he make out with him and even make him cum? For what?? For getting the phone? He couldve done other methods.