Actually, I would say that the reason as to why so many authors end up relying on the non-consensual tropes (blackmail, coercion, rape, etc) is due to the inevitability of the paradoxical nature of what people want from NTR. Most of the readers of the genre want a perfect heroine that would never betray their partner, and that only through the happenings of the plot, such a pure and pristine heroine is corrupted into a slut.
But with such a premise, it Inmediatly limits almost all manners of plots where she could be willingly cheating on their partner.
The most realistic case of NTR would be, something that happens all the time in the real world, is that one of the two people in the relationship would simply cheat for their own pleasure not intending for the affair to mean much of anything, then ending up falling for their new lover throughout the affair.
The problem would be that most people would not enjoy the netorare element of a game such a premise, because there wouldn't be a transition of corruption from Loving Wife/Girlfriend/etc to cheating slut. Instead, the wife/girlfriend/etc already was such 'bitch' from their very beginning.