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I have never responded directly to another poster before, but Alex, you may think that the fact that you're a graduate student means you're open minded and progressive, but you sound way more closed-minded than anyone I have ever met.
You've decided, without anywhere near enough information, that the LW is a "slut", has low self- esteem, that you know enough of her boyfriend's mental state to know that she must have slept with a large number of men and that her threesome was MMF, that most "normal" guys would react the same way, that she's ashamed of her past in her own right (rather than under the influence of his belittling), etc.
The fact that you use the term slut shows that you have some antiquated ideas about men and women. The fact that you consider her having had a healthy number of sex partners (which is an assumption-- for all we know she has had four and her boyfriend has only had two and that's why he's freaking out) a problem shows that you are nowhere near enlightened. The fact that you equate active female sexuality with low self-esteem does make you a misogynist.
The LW said that she was fine with her past, until this guy started belittling her for it. She has been the victim of emotional abuse for the past six months, and then you, brilliant, enlightened graduate student, come here and belittle her some more. Don't try to hide behind the fact that you're well-educated and well-read: you're a misogynist pig.