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I can understand why LW's BF would be a lil sick over knowing that his GF was in a threesome. I wouldn't, but I understand.
What I don't understand is his inability to get over it. I mean, at some point, you weigh the costs and benefits of holding on to that anger against being happy with someone you love. People grow and change. I once had to reconcile that a good friend of mine spent a couple of years being a bully. As someone who was bullied, it was kinda weird for a second. But then you get over it!
I wonder if BF has a history of finding his GFs wanting in some area, then using it to denegrate her. Maybe he has a problem being with women who are his equals. It sounds like the problem isn't just that she had a threesome, but that she hasn't always fallen in line with his rules. I wonder how many unfortunate people, especially women, haven't been able to follow his rules.
The best lesson that LW could teach him is to walk away (maybe like many other women in his past). He needs to learn that sometimes loving all of a person doesn't mean liking everything about them (or their past).
His loss.