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It sounds like it was a MFF threesome. This is such a powerful fantasy for a guy, yet so rare.
LW's boyfriend is jealous that he did not get to experience this and it is driving him crazy and irrational. I can absolutely picture this. She has been to the mountain top and he hasn't. Worse, unlike a male who would have to expend a lot of effort and (probably) emotional stress, she apparently just fell into it. She is being casual about idly taking a sip from the Holy Grail that he desperately was fantasizing about.
I think almost all the males who are writing letters judging this guy harshly would in fact also be messed up by this revelation. Maybe not reacting quite the way he is, but it would obsess them and obsess them no end. It's not her fault that she didn't think of the consequences of the disclosure, but now that it's out the effect of it must be dealt with. The best remedy is for LW to find another female so that her boyfriend can experience a threesome. Crazy as it sounds. only then can his head get clear and they can go back to getting their heads into the here and now.