I don't think that counts as true NTR, I mean if a couple isn't explicitly stated to be dating then they aren't a couple and therefore is not cheating, am I right? After all, all is fair in love and war.
Soap operas were infamous for the abuse of love triangles and the whole cheating-but-not-cheating way before NTR was cool. If the MC was female, expect to be "undecided" between all the male love interests, hook up one day, then feel "confused" and leave him the next day, then run to the arms of another one who has been patiently waiting, and so on, all while waiting for the handsome billionaire to finally make his mind and decides he loves her. If the MC was male, expect the femme fatale to try to steal him away from the main love interest, going as far as to seduce him just to spite her romantic rival. Our grandparents loved watching that stuff, it's nothing new.
The thing with anime is that, unlike in other media, great emphasis is put on virginity as a virtue. When someone loses his/her virginity it is considered "lost" to the MC, especially when tropes like "childhood friends" or "fated lovers" are taken into account. That's without mentioning cheating partners, that's another whole level and is true NTR. In western media, and also in real life, it is quite common for both men and women to sleep around until they find the "right" partner. It is not uncommon to meet your special someone with a body count around the dozen, and that's putting it mildly. Couples cheat, break up and get back together all the time, it's normal, it's accepted, and in some parts of the world is even expected. There is a saying in Latin America that says: "One doesn't eat the juicy watermelon alone" meaning that if your partner is someone with good looks, you shouldn't expect to be the only one "enjoying the fruit". Cheating is a fact of life, and it's striking when someone does NOT partake in those "games".