NTR is having a love interest stolen from the protag/MC by a rival in the present time. It is not seeing a female character, whether they be a LI or not, having sex with someone else before the MC was even in the picture. This is not "nonsense", it's the truth. What you seem to be offended by is knowing that someone you like has had sexual partners before you which is a very immature outlook because you can't expect women of a certain age to still be virgins. Seeing it happen is not NTR, it's just voyeurism because it happened in the past and has no bearing on the present.
Grace's flashbacks are categorically not NTR and people really need to stop applying that term to any and all interactions female characters have with other guys, particularly when it has no impact or effect on the present-day storyline.
Also, the devs haven't said there won't be any sex scenes with Grace and the MC, there just aren't any yet.