I like to read/watch challenging fiction. Most of my favorite anime and movies have quite tragic endings. Doesn't mean I don't like comedy, but I can appreciate more or less anything in media.
The point has been made before, but I think NTR causes people to rage because it hits closer to home than things like guro and/or tentacles because you're unlikely to run into that in your real life. Infidelity however is something that takes sex and rips it out of the comfort zone, and most people get confronted with it in some form or other.
The fact that the genre is so broad makes it inaccessible for most people I think. People are turned off because they ("accidentally") read a hardcore book and got traumatized. There are other titles in the genre that could be appreciated by a lot more folks.
Hey, maybe it might even encourage people in a relationship to spoil their respective other some more, because you'll know what's coming if you grow apart. ;)
Anyway here are some of my favorites:
Netoraserare!
Most people here know this manga and it's one of my overall favorites. 23 chapters and going and it gets pretty real. Involves a married couple where the husband has a cuck fetish. Dramatic, tense, emotional and confronting. Might break your heart but Haruka is a cutie, hey I'd try to steal her.
Regretfully I won't be able to read that one here anymore at least for now, it's more or less the only series I follow.
The Truth Is, We're Couples
Girlfriend Next Door
These two fall under Fakku subscription, both involve a childhood friends situation. I consider these to be pretty tame but very well done. The first one is pure gold, the second one gets a little more real.
I get that some people don't want the stories they read to end badly. This is why the NTR tag is put up front. But what caused this thread is simply people reacting on individual NTR chapters. Call me heartless but I always enjoy the people commenting there that forgot to read the tags.
I'm pretty warped myself I guess. I have a vivid imagination and I like to confront myself. I've had time to throw myself into the black pit that is hentai so not much really fazes me anymore, however I like that fiction has an impact on me in whatever way, and expands my views, character and mindset.
NTR works really well in visual novels, too. Still got an ending I found 3 months ago etched in my brain. But I knew what I was getting into. I had read the tags. :)