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And everyone else who says that the boyfriend is completely within his rights to be uncomfortable with a threesome and we are being too judgmental....
The vast majority of us are NOT saying he doesn't have a right to be uncomfortable. If he doesn't like the idea of a threesome, he doesn't like it. He is absolutely free to tell her that it's a deal breaker and leave. If she had written in saying, "my boyfriend dumped me because I had a threesome, isn't he crazy?" I would not be sitting here saying "yes, he is."
The point, as many people have reiterated, is that this guy has emotionally abused her for SIX MONTHS about the fact she had a threesome. He has nitpicked, withheld sex, made her feel ashamed, made her feel like everything she does is wrong, etc. It is based on this behavior that we are condemning him: he is acting in a controlling way, he apparently enjoys punishing her for long periods of time, and he seems to value the relationship not for the joy it can bring both of them, but for the power he can get in possessing her.
This is why he deserves to be condemned. He is absolutely welcome to leave the LW, go find a woman with sexual values closer to his, and have a wonderful, vanilla sex life with her, and most of us would not look down on him at all.