No, NTR means stealing. An LI having sex in the past before she's in a relationship with the MC absolutely isn't NTR, there's no stealing going on there. You can dislike it, I don't like to see scenes like that, but just because it makes you or I uncomfortable doesn't make it NTR.
Consensual non-monogamy isn't NTR either, having a threesome with a bro is at most swinging. Watching your bro fuck his girlfriend is voyeurism.
This site doesn't consider girl on girl to be NTR so long as both girls are a part of the harem.
You can't just call everything you don't like NTR, that makes the tag useless for people actually looking for NTR. NTR is a specific thing that requires a relationship to be subverted or destroyed through infidelity, with a villain trying to steal a heroine from the victim or protagonist.
Why are we talking about this again though?