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LW, I have a hard time feeling like if it hadn't been this, it would have been something else. Not even necessarily something sexual, just... something he arbitrarily decided was 'bad' and gave him the imaginary right to punish. This is so not about some blip in your sexual history - and, really, not exactly the most shocking blip in the world. I guess I wouldn't expect anyone's sig.o to join me in saying "Hey, good job on the MMF," but it's still beside the point. That kind of abusive behavior does not have an outside cause. Your boyfriend did not freak out because you had a threesome once upon a time way back when; he took the threesome thing as an opportunity to freak out - and in such a way that gives him all the power in the relationship, that has you begging for forgiveness for something that (say it with me now) was not wrong.
From the letter, you sound like a vibrant, confident girl, except for the parts where you're regurgitating the bullshit this guy has been drilling into you. I would be very surprised if a survey of his exes showed this type of judgment and treatment to be an anomaly from him. I'm very glad Cary was so uncharacteristically definite with his advice, because you really need to leave this guy six months ago.