Lol at using urban dictionary as a source, anyone can write whatever the fuck they want on there.
I'm also not sure why some people think characters need to be in a relationship for it to be considered NTR when there's literally thousands of NTR-focused doujinshi that involve things like relatives, someone the protagonist has a crush on, friends, etc.
As for Cabin Corpse, I'd say from the point of view of the story, it would be hard to truly call it NTR since the MC doesn't seem to care and Susan and Michael are married so it's not surprising that they would have sex.
That said, for someone playing the game I can see how it feels like NTR, especially for Charlie. You do quests for the girls and get to know them and hopefully care about them as a player, and then you're rewarded with the MC spying two of them have sex with Michael. The passive protagonist powerlessly spying on his love interest having sex with another man (who had to make zero effort for it to happen) is pretty much the textbook NTR scene, it's only missing him jerking off while crying.
With Susan, it always seemed like it was leading to the MC eventually stealing her from Michael but the Charlie + Michael stuff comes out of nowhere and the whole "I'll just wait and take pictures to show Susan" makes no sense, especially considering Susan could understandably hate the MC for not intervening and letting it happen. All of this is made worse by the fact there's only 3 female characters, the MC's weird voyeuristic tendencies and the lack of player agency.
