How does the NTR work here?
In practical terms? You chat up / date / fuck one particular NPC until they like you enough to agree to start an exclusive relationship with you.
Unfortunately, you're living in a hentai city. There are more rapes in any given month than there are citizens, to the point that not only do the police explicitly ignore reports of rape, they're notorious rapists themselves. Unless you've got a very
particular playstyle that doesn't involve being outside or interacting with most NPCs (ie: if you become Olena's live-in sissy maid and spend every waking hour cleaning her house), it's very difficult to avoid getting raped by
someone over the course of any given week, plausibly in public.
Despite the omnipresent power of the hentai force ensuring that avoiding rape is more about luck than skill, NPCs will still consider any sexual contact with other NPCs to be 'cheating'. Every time you get raped, there's a chance that it gets back to your partner; this is presumably more likely for when you get publicly gangraped as opposed to privately raped in a bathroom or something. Once they find out, they'll break up with you, kick you out of their place, and proceed to hold a grudge against you.
Effectively, this puts the player character in the role of the NTR plot girlfriend (even if they're male and dating a female); you go out and get raped by the Ugly Bastard, and then your partner finds out and dumps you for your statistically-unavoidable infidelity. To opt out of this, don't offer or agree to start a relationship with anyone ever.
As a sidenote, the NTR isn't entirely one-sided; certain slutty NPCs (anyone you can hook up with at an establishment that's open at night) will cheat on you on their own, and you might even be able to catch them in the act. This doesn't stop them from breaking up with you if they find out you've been unfaithful, though.
tldr; the NTR system is a garbage implementation of real-world sexual ethics in a city that runs on hentai-logic, and it's so bad that you should just avoid relationships entirely.
