I wouldn't classify those as NTR, nor would I classify it as such if the MC is the one who somehow causes the love interest to sleep with someone else. I wouldn't particularly want that scenario, but from the MC's standpoint it definitely wouldn't be NTR in my mind.
I wouldn't count a father sleeping with a mother who is a love interest as NTR, since that's an established relationship that obviously has been going on since before the MC was born. Although if the father was out of the picture because he had left, then suddenly returns after the MC has developed a romantic interest in the mother, I could see that as NTR.
If the MC traveled back in time before the parents met, and romanced the mom, and then the future dad suddenly appeared... I could kind of see that as NTR, even if it would create a paradox or invite the appearance of the Temporal Variance Authority. I kind of want to play that game though. A Back to the Future variation, but Marty is actually interested in boning his Mom before his dad can.
For a lot of other stuff that gets classified as NTR, a broad definition often gets used because we don't yet have a good term or tagging system for "if you don't like NTR, this will probably bug you too" or "this seems to be leading into NTR territory". Other tags get used somewhat liberally, like incest or rape. A strict definition of incest might be close blood-relation, but in practice it gets applied to step family or adopted children and the like. Similarly, the rape tag may get applied because someone tries to rape someone, but the legal definition of rape does not occur. If someone is looking to avoid rape-y stuff, they'd want to avoid that though. The tags exist not just as a "this has the kink you want" service, but also as a content advisory.
The tag system could use an overhaul, but then we'd potentially end up with an obscene number of tags to describe various obscene acts and kinks.
