Slut porn and prostitution is NOT ntr and yes the protagonist has to give a shit (either liking or disliking) for it to be cuckolding, in this case he does not.
The girls are neither Billy's loved ones nor are they "taken" away so by your own definition it is not ntr.NTR:the main protagonist's loved one(s) are taken or seduced away from him and the heroine might be willing or unwilling.