One: we can't because the NTR tag is for everything.
Two: pretty much like above.
Three: "I don't think it's a big turn-off for most people if they find out the NPCs are cheating on relationships, certainly not to the point where you need to warn people." Are you sure? Why don't you go and check "Dream Of Desire", "The Family Secret" and other thread where mod and staff had to put a big, beautiful red warning about NTR matter everytime people went completely nuts and started to say everything, everything? Then come back and tell me what you have found.
Now, if you want to complain again about this, open a thread in "Features Request" and ask for this, though like I said to another one who asked the same thing, the staff already rejected this. So deal with it.