samjohn hate fucking whilst sam yearns for dean and john yearns for mary. it's not consensual because if sam didn't think it lowered the tension whilst traveling, he would deck his father in the face (like he previously had tried). john figures out sam tries to think of dean when hes inside him and gets bitter over it, not about sam wanting dean but dean being there alive for sam to want. he can't relate, he can't have mary besides in his memories. he fucks sam like he's punishing him for having hope and having faith in dean, albeit it he's the one that led to sam seeing dean in that light.
maybe at first john thinks sams yearning for dean to save him from this situation, and john huffs at this all irritated sams acting like he isn't a willing participant (ignoring how loose the term willing is in this situation, his own underage son for christ sake.) but when he realises it's dean he wishes was pounding into him, bruising him and whispering bitter words into his ears he's amazed.
