The important thing is that the two of you have been dating for 18 months and discussing marriage. In addition to which it really doesn’t seem like the two of them have more than a flirtation, since the one time they met, she brought her husband. I completely understand your distress that he lied to you about this meeting, but now you have to decide how to proceed, and my advice is that you try to put yourself in his shoes. Empathy is going to help you understand his point of view and make a decision about your own behavior.
He’s probably got a crush on her, but he’s in love with you. Even if he marries you, he’s not going to be dead — just married. In other words, he’ll still find women attractive. He’ll still flirt. And he’ll still crush. This doesn’t mean he’s going to be unfaithful or that your marriage will be in trouble. The question is how much of this you can be okay with. I’ve seen couples break up without any cheating because one person is so worried the other will cheat, that that worry becomes the breaking point. This is what you want to avoid.
I think that for now, you have to back off your worry that he’s going to cheat. You’re in a committed relationship with him. Make the relationship your focus — not the obstacles that you imagine. Bring your A game and consider his feelings for her competition — not a straw to break the camel’s back. If you put him on the defense, you’re in a weaker position than if you compete and win him over. And consider that it might be better if you keep quiet about what you’ve learned by going through his phone, and give him the opportunity to disclose it to you when he’s ready. If you do, you’re both in a much stronger relationship position than if you accuse him of lying and he accuses you of snooping.
I hope that helps!
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