Bad Endings are a wonderful idea, no matter what form they take (NTR, deaths, breakups, etc.). But I do think that if you decide to add it, you'll have to put a warning for the "mentally fragile" people. For example, if a certain choice branches into a bad end that has NTR, put a NTR warning in for that specific choice, or make it extremely obvious that the choice will lead to a bad outcome. It might spoil the surprise or endings for some people, so I think there should be an option in the settings to toggle the warnings on or off depending on the player.