i did not see that post but just to clarify you some points because i think you are confused as some other people by what this means, since this fórums is not really using it as intended.
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Netorare (寝取られ) is a sexual fetish designed to cause sexual jealousy by way of having the heroine indulge in sexual activity with someone other than the protagonist.
For a game to be classified as Netorare the following conditions must be met:
1. The heroine either must be romantically involved with the protagonist, a potential romance for him or of someone that the protagonist shares such a close relationship (sister, mother, osananajimi etc.) that jealousy is a factor.
2. Related sexual events are explicitly shown to the player or later described in detail.
This does not include relationships of heroines that are outside the route you follow. Explanation: Route 1 is for Heroine A, the protagonist is romantically involved with her. During that route, if she sleeps with other characters than the protagonist, it would be NTR. But during Route 2, which is about Heroine B, where the protagonist has no romantic relationship with Heroine A, her having sex with others wouldn't count as NTR.
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main heroines... (meaning main love interest of the mc in the game) and dont twist my words i never said fucked in the past, i said fucked in the future, when they may have already have feelings for that heroine or see her/him as a potential romance, that is showing interest in mei for example) of course that includes people close to the mc --> chilhoodfriends, moms, sisters etc unless the mc dont have feelings for a heroines its all valid, as you can read it --->
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(also as we have see in that description shows perfectly almost what this game is pointing out, mei route or wife route with the only problem that in both routes the mc can have feelings for both or in case of mei see her as a potential romance, so have that in mind when you read the last description, an example can be if you show interest only in the wife and pay no attention to mei, you seeing her fucking someone else is not ntr since you are not inrested in her in any kind.)
Regards
(maybe this fórum will need something like vndb use, a feature or some kind of thread with a explanation of each tag, since some people get it wrong and confuse others)