@CheerTheDead that should be borderline hentai and still count as Ecchi then. Aki-Sora has one scene of sex/almost sex per chapter and it's tagged as Ecchi and not blocked (the actual manga was even banned in Japan for some time)
Pretty much a hentai, except author try to make characters behave somewhat realistically. It gives of a feeling as if author drawing some of his own teenage-years experiences.
I don't think it's NTR since they both not love each other. They just have sexual relationship, and that's all.
You just feel it's NTR because you assumed or feel like they have a mutual relationship with each other in a sense that they're like a couple.