I understand this thread is old, but just in case this is helpful to anyone else searching for this. The main ways I've heard of "reverse NTR" definitions is mostly this:
1. NTR is a general abbreviation for several subgenres, netorare being the most popular, but also netori, netorase, etc., although sometimes other abbreviations get used like NTS for netorase are used, but yeah, at least what I've seen commonly brought up. Considering netorare is typically about having love interests (usually females) potentially cheating or outright stolen from you by other side characters, the "reverse" would mostly be in context of specifically netorare (and not netori, netorase, etc.). It would be the opposite in a sense that the main character is in a relationship, and being tempted or stolen by other side characters (again, usually females). Although, it's not like this is universally accepted either, that's just what I've seen in most cases.
2. Without the japanese origins, a number might consider this upcoming definition to be pretty close to "cuckquean", but the specifics are different, especially since NTR is not gender specific (the origins of the word doesn't account for gender). And that brings me to the second definition some people mean when they use "reverse NTR". You could play a female protagonist where the male love interests can be stolen by other side characters (usually females since it's "reverse") which, again, to the best of my knowledge and it would still technically be considered netorare, although some, used to the tag typically used for games in a very gender specific way, feel the need to add "reverse" to help locate games of the opposite gender, and thus call that "reverse NTR".
Those two I feel would be the most common definitions you'd come across when people are looking for "reverse NTR". And there are several games in this category on here, but they are very rare to find. Examples would be:
Confession Game This is personally one of my favorites and I wish there were more games like this, plus saw someone in this thread already mentioned it. This game will mostly fit definition 1 of what I said. The problem is it doesn't exactly fit the definition of cheating (except for the extra after game part) but gets super close.
Trials of the Succubi This is probably another good example of what would be considered a "reverse NTR" game fitting definition 1. The male protagonist has a significant other they are about to be married to (you can choose to have female fiancee or male fiance). The goal of the game is mostly just to make it through to the end of the day without being stolen by the succubi you will be encountering. So if not obvious, this fits because you're ultimately trying not to be stolen or tempted into cheating.
The Siren's Song This is another one that might be a good example because this game you could argue can include this tag, and can potentially work with both definitions I gave, but probably definition 1 more than definition 2. The key in this game, is it has two main protagonists you switch between: the female protagonist and the male protagonist who are husband and wife. You play from wife's perspective one day, then switch to husband's perspective that same day, then move on to end day event, and go to next day. Both the husband and wife will be given lots of temptations to cheat on and possibly lose their partner with other side characters when the story concludes if I recall correctly.
Camgirl Confessions This game is more an example of reverse NTR fitting definition 2. Warning, explaining will probably spoil the biggest part of the plot in this game, so don't read if you care about this game's plot.
Finding these types of games (by both definitions I used at least) is actually quite difficult since they're so rare. I may have come across like 1 or 2 others (fitting either definitions), but I don't remember the best. At least for the ones I listed, all of them are completed (but some still get updated). I'm actually a fan of games like these, so yeah, if people have found others, I'd love to hear other suggestions too. But at least, in case it's helpful, here are my suggestions as I didn't see too many listed here when reading over this thread.