Unlike most tags which denote an event happening, NTR is more of a general term to define a genre. Netorare can be thought of as the anti-romance, it is the story of two people growing apart due to the actions of the antagonist. However, they don't have to be in a formally committed relationship that we would recognize in the real world, in terms of storytelling the "will-they-won't-they" main romantic interest(s) or crushes of the protagonist are generally the same thing to wives or girlfriends in terms of story-writing. To simplify it, a "heroine" is any girl the protagonist has shown a genuine romantic interest in but may or may not have taken that leap into asking her to be his.
The second question is how is it portrayed? This is kind of like horror when asking "is it scary?" Everyone is going to feel different, some can sit through the most hardcore netorare and not feel a thing while others will lose their mind at any sex not involving the protagonist. One must ask "did the author intend to evoke jealousy in the audience" and assess it regardless of whether or not the author succeeded in those goals.