Don't get my wrong, I don't view any character as incorruptible. But I believe NTR susceptibility differs from person to person. Hence we have the 3 types of NTR variations to signify just that. Now imagine this, you just got off reading a manga which had a character that doesn't even regret that she gave up godhood for being with someone who wished for her to stay with him forever. As the series progresses you see her morals solidifying to the point that she gives a new definition to the term "fiercely loyal" but still remains shy throughout.
But then you see a doujin that shows her in full blown NTR-A slut mode. I mean don't get me wrong, but the way Bledandy is projected, it would take a colossal amount of NTR exposure to have that kind of effect on her. Most probably her mind would have to be broken because in the present mindset she just won't go down that road.
So for me, Beldandy went from "The perfect woman I would take home to my parents" to "The perfect slut I would take home to my underground dungeon" What happened in between is a big mystery. A HUGE gap like that really messes with your immersion. Long story short, if you are a NTR fan that gets off on the story and the whole circumstances that lead to the fall of the woman then it will take a while to warm up to a scenario like that because your mind is gonna have to fill in the NTR blanks.