I mean.. if you wanna get technical there is more than two types of NTR. It should really only be used when there is a definite effort on the part of the antagonist to steal the MC's love interest away, but usually I see it used here for any instance of infidelity regardless of who is being affected or how.
Here are the basic NTR subgroups that I've come to know:
[Netorare - type A]: where the MC's partner intentionally cheats on the MC and is a willing participant in the sex from the start.
[Netorare - type B]: where the MC's partner doesn't want to cheat on the MC, but their mind is changed from the pleasure of sex (usually the partner is raped in this scenario).
[Netorare - type C]: where the MC's partner at no point wants to cheat on the MC and is unwilling throughout the sexual encounter (almost always rape).
All of these can done with either a boyfriend or a girlfriend. As long as it is about making the MC/Player jealous of someone trying to steal their love interest the definition fits. They all generally suck heavy balls and I wish I could fine-tune my searches to filter them out.
Then you also have:
Netori: where the MC tries to steal away a love interest from someone else, usually through coercion or rape. It isn't netori if they are having an affair without the partner knowing.
Netorase: generally loosely defined as sharing or cuckoldry. It is not swinging! The MC is willing to see their partner with another person either because a) they get something out of it (like money, or arousal), or b) because they trust the person their partner is having sex with.
All the Neto- words (apart from netorase) have in common that the the person risking losing their partner knows that this is happening, so it can't be used if no one ever finds out. There needs to be a jealousy aspect involved.
Anything other than this should just have the [cheating]/[infidelity] tag which is not about humiliating/traumatising someone, or the [swinging] tag where both parties have sex outside of the relationship (if only one does it it isn't swinging, then it's closer to harem or netorase).
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