I know most people are saying "Leave her alone, she has someone else", and are taking it too seriously, but I see it from a different perspective.
1. She's not married. Boyfriend/girlfriend relationships aren't serious in the least bit, nor are they set in stone if there's no rings. Like someone before said, no rings, no strings.
2. SHE is the one cheating. It's her problem, not his. Everyone's giving the poster a hard time saying "I hope your wife cheats on you", but he's not really doin anything wrong. Don't forget, the girl could've said no. There's two sides to this. To go along with #1, if she's going out of bounds, then the relationship isn't that serious or doesn't mean much to her. Her problem.
3. I don't think he plans on staying with her. Rightfully so, she can't be trusted. Just a person to have sex and fool around with.
To the poster: take some karate classes. That way if someone's boyfriend finds out, he won't be able to touch ya
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Speaking of which, boyfriends, unless your woman was FORCED, don't get angry at the other man. Instead, redirect your anger toward your girlfriend. Ultimately it was her decision to cheat.. or better yet, just walk away from the relationship! Quick, easy, clean!