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It is amazing how insecure people are. Not just your bf, but many of the respondents. Madonna/whore complex? Choosing to be with someone cannot be boiled down to ONE vein of thought. That is absurd. Things are not equal, but that does not condone his behavior.
Please, LW, I beg you to get out of this relationship now. He will not get better. Trust me. I went through the same thing you did, the jealousy, petty comments - the counseling, the ATTEMPTS at counseling. Please believe me when I say this is all a manifestation of HIS insecurity, and there is nothing you can do about it.
The main problem here is that he will not let this incident go. And, I can tell by the language of your letter, that you are starting to believe him - and you have started to believe that you are a disgusting person who did a disgusting thing. Don't listen to him.
I can probably tell you what's going on in your life right now: every few weeks or so, he will find something new about you that upsets him, or some comment you said, or some outfit you wore, or some co-worker or waiter or man on the subway who was looking at you the 'wrong way'. He'll have perfectly 'logical' arguments as to why that reflects negatively on your character, and argue in his defense, "Well, how am i supposed to know you're not like that, considering the whole threesome thing?"
And you, instead of getting angry at him, will try to be caring and understanding, because you feel guilty. At the same time, it will become increasingly difficult to be intimate, because now you are afraid of expressing your desires, or fantasies, lest you be judged. You will get paranoid every time you speak to a man, ANY man, even your 45 year old, balding, pudgy boss. You will think twice before you speak and look twice at your clothes, for fear of being 'accidentally' seductive. You are being constantly judged and criticized, and the sad thing is, you will start to think he is right.
Please don't. He does not appreciate your compassion. He sees it as weakness and exploits it. I will repeat what my bf told me, after which it became so easy to leave him - "The thing I couldn't respect about you was that you always tried to be understanding after I acted the way I did."
So, out of the pig's mouth. He doesn't respect your kindness. He doesn't deserve you. Leave, and be with an adult.