I sorta consider it to be a combination of the two. The mom acts more or less like a typical hot-wife. But the son experiences the cuckold feelings of angst/betrayal/whatever. Maybe he wants to fuck his mom, maybe its more a 'stolen love' kinda thing, maybe its just the humiliation of the old schoolyard taunt* actually being true. Whatever it is, I think a cuckson has to feel something along the lines of 'how could she do this to me?!' in order to be a part of that genre. And I also think that is a big part of the masochistic appeal: a mother's love (even if its not romantic love) is supposed to be one of the few absolutes that someone can count on. And a cuckson loses that, or at least sees it diminished.
*Side note: never really understood how 'Dude, I fucked your mom' is supposed to be an insult to the son ... to his father, yes; but to to the son? It shouldn't bother him too much. Hell, given that most moms of teenage boys are in their 40s or even 50s & typically no longer in the best of shape ... its more of a win for her to attract a teenage stud (conversely, that guy probably shouldn't be bragging all that much).