You can bang (or be banged by) anyone/thing in the game pretty much. Only a few can be called a "romantic interest", though. The only specific NTR I came across was when I was eating lunch with Robin (a chosen romantic interest and kinda the "default" one) when my PC was yanked away by Whitney (the school bully) and fucked. When my PC got back I had the option to tell Robin that Whitney fucked my PC and it was really good or lie about it. Also, when helping Robin sell hot cocoa/lemonade you can go with some cops to "pay the permit" for Robin's little business with your characters' hot little bod or go off for more water/milk and pay for it the same way, but Robin never seems to suspect anything "untoward". Robin will catch you stripping when their confidence is high enough to go to the titty bars, but that only drops their love for you a few points that you can recover pretty easily.
Everyone else pretty well accepts my PC as a cock-hungry bitch whore ready to fuck anything and everything with a dick or otherwise. You can even have (for now) two romantic interests, but still screw anyone and anything else. I'm personally waiting for The Ivory Wraith to become romanceable then swap meek little crappy Robin for the hot & horny thousand-year-old ghost.
At this time, most of the NTR has been "unwritten and uncaught".