Pacifier

Most mangas accused of netorare, while may indeed contain elements of it, often have the protagonist "take back the love" (Ai wa Torimodose?!). A popular method for immersion is simply pissing off the reader and then granting the satisfaction of revenge. A very effective way to piss the reader off is to steal (as in steal the relationship, not just physically capture) the protagonist's lover which, without the revenge part is NTR. I guess it's easy to assume any manga which provokes frustration or creates some feeling of loss is NTR even though it doesn't involve any 'cheating' on the protagonist (which tragedies do a lot and are aptly tagged).

I believe hardcore NTR preserves the despair to the ending, never giving the protagonist, and anyone who empathizes with said character, anything but frustration. While it's true this seems to be a solid rule, it depends on how sensitive the reader is to this sort of psychological bullying and how 'satisfying' the outcome is. If the reader decided to stop reading and miss the ending, then what was read could qualify as 100% NTR.

I agree with thearux though, the Drama (Tragedy too in some cases) tag pretty much covers and it, just as Romance covers the disputed topic of incest. That being said, Drama is too broad and many blurbs make a point that incest will happen (if not the cover page itself) while NTR is almost impossible to see coming without spoilers (since the protagonist is implied to 'lose') unless there is little to no plot (aka porn). Perhaps a 'Bad Ending' tag is in need? I jest, there's no real concrete solution.

However, here's something worth taking with you:

Devoting feelings to a plot is asking the author to play them like a harp. If you aren't prepared to lose, don't invest.