A year ago my boyfriend and I were talking about being in an open relationship but only on his side (ie. he can screw around but I can't). I said I wouldn't consider it cheating if it's just sex and if he was gonna be honest to be about it. So we talked about ground rules like it can't be over overexcessive and he has to tell me when it happens. A good while later I find out through being on his phone that he did ending up fucking a girl, but didn't tell me about it.

We get into an argument, but he ends up saying he's sorry and that he should have told me. I'm kind of mad still but should I be more understanding? Not exactly sure how to react and if I can trust him to continue this kind of relationship anymore.