The NTR tag is for, and only is for, when an MC's love interest fucks someone else for the purpose of making the MC jealous.
It doesn't include things like polyamory, pimping, cases where a woman just has a boyfriend, or anything except jealousy-play by an LI of the MC. It doesn't even include cases where the MC NTRs other characters.
The tags are meant to be specific and informative. For example, if you don't like cases where the MC is watching someone else get fucked, then you'd want to avoid the 'voyeurism' tag.
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That jealousy is the important part.
I'm going to play this game, and see this scene, and decide, but if the poster is following the rules and there's no jealousy-play in the story, the tag shouldn't be there, as a heads up.
Yeah, that's not the NTR-tag. That's just straight up voyeurism. If the eventual goal is for the MC to NTR the giantcuntboyfriend, then that's still not NTR-tag.
We just call that polyamory. It's a thing.