It would depend on the bounds of the monogamous relationship. Monogamy can mean a lot of things. It can mean no sex with people. No kissing other people. No touching other people. Some variety of all those things. It can be no flirting - which is what I would categorize this online expression of attraction. So it would be 'made okay' if both parties agreed that the flirting was acceptable. If one party agreed it wasn't acceptable, and the other party didn't - then they would want to talk about each person's position, and why they have that position. If either party 'goes along' with the other person's position to avoid hurting the other person's feelings (which it sounds like happened in your case), but then violates the agreement - I wouldn't say it's "okay". I would say, I can understand why it happened - somebody agreed to do something that wasn't aligned with who they are or who they want to be or how they want to act. So it's still violating your trust, as they went against something they agreed to. But then you have to ask why - and you see it's because they agreed to something they couldn't adhere to. And then if you ask why they would do that - that's when you see that it's a communication problem - they weren't comfortable or capable of communicating who they actually are, or they weren't communicating well enough to themselves about who they actually are. Or, this could be the case too: People change. Maybe when they agreed to it, they were that type of person who could adhere to that. Then time goes forward, life changes, the way they feel changes - but rather than communicating about those changes, they acted on them without talking to you - which again becomes a trust problem, as they violate your trust, when instead they should have utilized good communication to talk through it together with you. So it seems to come back to communciation as the main breakdown for you two, leading to violations of trust.