@Lilliwyt:
>Urban dictionary
Ummm, no, just no, in fact you can have cheating without it being NTR (one-night stand, swinging, etc) , that alone already make the 'definition' you linked wrong.
The real key of NTR is right there in the term.
寝取られ
寝 = sleep i.e. while unaware
取られ = taken
If it's just cheating but the person go back to their spouse, it's not NTR. The point is they're taken/stolen while the other person isn't aware until later, not just the sex.
I'm not arguing that this chapter is NTR, but that NTR does not require the characters to be in some kind of official relation, just the feelings + audience sympathizing with the POV character.