tl;dr: sometimes women (and men) will attempt to get into a position where you are simultaneously dating and not dating them; as it gives them advantages.

This video got recommended to me on youtube. Normally wouldn't watch but clicked through. The guy shows his text messages with his girlfriend who slept with another man. Before she did she said they were on some kind of break. I advise you don't watch the whole video as I didn't; just skip to bits where you can read the texts. https://www.youtube.com/watch?v=XbL2gTjAM50

The girlfriend got into a position where she was both simultaneously in and not in a relationship. Like Schrodinger's cat she is in both states until an observer forces the wave equation to collapse.

Unlike the cat, a person doing this will resist the observer who attempts to force the state to collapse. They will say 'we've been through this, I'm not explaining again' while keeping it vague. They will close off communication by saying 'stop texting me', then later if the branch swing fails they will return and say they never left you, they just needed time alone. They will say its none of your business that they spent the night staying around their male friend's place, because they were on a break, but you can bet they will be furious if you did it. They will lie about sleeping with another guy, showing it matters, yet when caught say it didn't matter as you broke up.

The best protocol is never to go 'on a break'. Just break up, or stay together and fix the problems.

But sometimes you can play all your cards right and this still can be done to you. In university I slept with a girl twice over a 10 day period. Call her Alice. She said we couldn't date as she was best friends with my ex. I said fine by me. I bumped into her 5 days later at an event I was DJing at. She was ignoring me all that night, and went back to my college dorm area to sleep with another guy that night. I was a little put off as I liked her, but no harm done. A week later I slept with another girl and word went around the grapevine. Slowly over the course of 5 months people started to ostracise me more and more. I didn't know why and nobody would acknowledge it when I talked to them individually, but in groups of people they would pretend I no longer existed. It actually was pretty horrible to go through. Eventually I discovered why, but only after graduating. Alice was friends with these people and convinced them all I had cheated on her. We had only slept with each other a couple times, and she slept with somebody else before I did, but because I never collapsed the wave equation and she didn't know that I knew she slept with someone else: she was able to convince herself I cheated on her. By the time I realised what had happened it was far too late to fix the reputational damage, most people just believe group social consensus and don't care about true truth.

So depending on your machiavellianism - and there are long-term greedy reasons to not be short-term machiavellian or dark triad - you can either create or abuse or defend yourself from these situations.

I have seen guys on relationship subreddits say that their girlfriend of 9 months who they were spending 3 nights a week with just slept with another man. And redditors have told them "you never had the 'are we exclusive' talk, so she didn't cheat on you, you were only sleeping together". I somehow doubt the same would be said if a male fucked someone after giving a woman 9 months of loyal commitment. Its a ridiculous technicality loophole. The only winning move is to get explicit about boundaries and collapse some wave equations.