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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.
I agree that the guy is not reacting well, and may very well be acting abusively. He may very well be a complete jerk. The decent, mature thing to do would be to honest, acknowledge that he couldn't handle this, and leave. All this is true. I'm just hesitant to CONDEMN him on the basis of a few slim paragraphs. I'm sure the situation is complicated, and jumping to conclusions seems unfair.
I also agree that double standards and hypocrisy are noxious, and that men who think it's their right to dictate sexual behavior to women ought to put a sock in it.
But while you may not fall into this camp, there are plenty of people on this thread saying he has no right to judge her, maintaining that sexual history is no legitimate concern for a potential partner, intimating that any man who has reservations about a woman with a lengthy sexual history is a knuckle-dragging patriarch, etc. This is unfair. Some of us are just old-fashioned, prefer to live our lives according to certain standards, and want partners who do the same. As I said, people with different sexual mores can do as they please. I'm not trying to force my values on anyone, but don't tell me I'm a self-righteous asshole for having them. People have as much right to reject others on this basis as they do on the basis of Republican party membership, fondness for Justin Timberlake's music, poor taste in interior decor, etc. We all make a million judgments in our romantic choices - if we're so hostile to hypocrisy, we ought to acknowledge that.