There's no one-size-fits-all answer for this.

If I can, though I'd like to offer you a different vision for relationships than the monogamy-in-thought-and-mind-and-heart-and-body-or-else model that dominates in most of these subs.

My wife and I are both on our second marriages, and happier than we've ever been.

In each of our previous marriages there was deception and cheating and all the misery you're going through now. The natural reaction to which is, "I can't trust you anymore. Out you go."

But we were friends for seven years or so before we got together, and we talked about our feelings and our relationships together constantly, and one of the things that we discovered was, we were discovering that there's more intimacy for us in actually sharing such moments as the one your boyfriend had: a hot person attracted him, showed an interest. That happens to me and my wife a lot: and our reaction begins with, "Of course they were attracted to you: I find you incredibly sexy, why would they not?"

It's not always comfortable to know your spouse / partner found someone else attractive and did the natural thing and flirted a little, or even a lot. But it's REALLY consoling to know, "He / She is going to tell me everything."

Why don't people tell each other immediately about this kind of thing? Usually because of guilt or shame. Even if it doesn't go anywhere (your boyfriend belatedly toed the line), a person can feel he's failed some very difficult standard. Overcoming that takes real personal strength in both partners.

And that's because it requires a lack of mutual shame: you don't give those million signals that "You fucked up and now you're gonna pay." It takes enormous self-confidence. Most of all it takes enormous trust in a relationship and trust in our partners that we spent almost a decade building before we even kissed. We knew we'd tell each other the truth, because we knew that as soon as we started lying to each other, we'd be right back where we were before, unhappy, mistrustful, and most of all, lacking the intimacy that makes us happy together.

Nobody can tell you what you SHOULD do in this case except yourself: it's your self-image and your admiration for your partner that's at stake here, not mine and not Reddit's. Do what's authentic to you.

But there's more than one way to construct monogamy. There's more than one way to structure trust.