Assuming you're genuinely, genuinely asking, I think this question deserves a genuine answer.
To answer how someone would get a boner from such a depressing premise, I would have to answer why someone would get a boner from NTR first, which could vary wildly depending on each person.
So, from my experience, I can no longer make the connection between myself and an MC who fucks 20 women while my dick is in my hand. With NTR, I don't have to pretend I'm not fapping and am no longer disconnected. Not that you wanted to know that, but now you can understand when I say: This depressing premise further justifies as to why the MC would choose inaction. That is, the question as to why the NTR content is taking place is further removed from the more pragmatic or weathered individuals (500M dollars outstanding...).
That is one example as to how one would get a boner from such a depressing premise. Some people need justification, others need to feel pain to get the rush, and others may just have submissive personalities. The mind is an amazing thing my friend, and there's some really kinky motherfuckers lurking around here...