Fighting one injustice with another injustice leads to further injustice. It is not maths wher two times negative equals plus. Not saying I am an expert when it comes to relationships (far from it), but I know that revenge leads to dispair. This old saying "If you look for revenge, start by digging two graves" holds true in any case... if you go that way, you are digging graves for both your hearts. Tell him how you feel, and that you want to rearrange the terms of your relationship, because you cannot go on like this. Mistrust is not even a good foundation for friendship, and you cannot build a successful intimate relationship on it, I am sure. And I hate to say it, but in my oppinion, your snooping after him is equally as condemnable as his intense flirting. If he has really given you reason for you to not trust him before this incident, we wouldn't be having this topic here, you would have either brought this up or dumped him. When you agreed on both being able to flirt, apparently you both had different agendas as to what flirting includes. That's why you need to rearrange this "contract". Without wanting to sound too much like a lawyer, but the misinterpretation of an unclear or not adequately defined term cannot be held against the person who did not set up the contract. It is not clear who that person is, and do not get me wrong, I am not justifying your partner's behaviour in the slightest, but you should see it from his perspective: He thought what he did was well within what you agreed upon, and while I strongly agree that he should have been able to tell that what he did went too far... being able to tell what you do is wrong and actually do realize that it is wrong is two different pair of shoes. Long story short: Talk, talk, talk. Also, tell him that you are hurt by the fact that he now feels the need to protect his computer with a password, and that you will have no need to snoop on him again if both of you agree where the limits of "harmless flirting" are.