it’s basically for when Rarity’s would-be love interest
is falling for someone else and Rarity hates it.

It’s a genre of adult manga or hentai. It should be defined as CHEATING/Unfaithful/Adultery.

It is too often INCORRECTLY defined as “cuckholding”. However true readers of the genre know that cuckholding is just a sub-element and a fetish. A fetish where one likes to watch their significant other, having relations (usually sexual) with another person.

In NTR(netorare) however the the a bf/gf/wife/husband Do Not have to make a appearance in the story whatsoever. They often Do Not Ever find out about the betrayal. In some ntr the cheated on individual may in up in a situation where they are forced to watch there significant other cheat on them. To go slightly further. Some ntr is known as NETORI. This can also be defined as Cheating. Ecxept it’s from the point of view of the individual stealing away the heart of someone already in a relationship. Once again the gf/bf/wife/husband never has to make a appearance in this type of story.

For the illiterate/Tldr;

Netorare is just Cheating NOT Cuckholding. The individual being cheated on may never find out or even be shown in the story.

A sub-element at best seldom/sometimes used, Cuckholding is a fetish. About an individual’s desire/want to be cheated on by their significant other.