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Get the hell away from this emotionally abusive man that is going to make you pay every time he finds out about a mistake or just something he doesn't think a "lady" should do. He's just got this threesome to pick at now, but even if you had never mentioned it he would have found something else to lord over you. Also, it's possible that initially he made you feel so good about yourself because he put you on a pedastal, but you just didn't know he put you there so he could knock you off it later.
If it had just been his initial reaction, sort of an "how could you do that" sort of thing then let it go, it'd be fine. I'd gone through this with my now husband, I'm a don't ask, don't tell type of gal but every so often one of his dumb ass friends would let some previous sexual exploit out in the middle of a conversation and then I've got new mental pictures and there were occasional past sharing because it was relavent where he would say okay, too much info, no more sharing please. But it never took longer than an hour or so to get over it and the same for him and we wouldn't pull it out in future arguments as a weapon. Our love was enough to get over any initial icky feelings and decide that they were irrational and put them away. It was actually good that he and I were more on the kinky adventurous side because it's made our married sex life a blast.
But to the LW, you need to learn to grow a spine. There is no way that he should have been allowed to berate you for 6 months, to withold sex, even the therapy is too much, someone shouldn't need threapy to accept you as you are. Even if you look back and think what you did wasn't that great or you aren't that proud of it, he's got no right to use it against you. There are things in my sexual past that I wouldn't do again if in the same situation, but those were my learning experiences, sometimes people need to do the thing to decide it isn't for them and there is no way in hell I would have ever let my boyfriend/husband make me feel bad about them. Stand up for yourself, you are not a dirty whore, while some people may think what you did was immoral, so the fuck what, you did it, you know you're not a whore so stop letting him treat you like it. Tell him to grow the fuck up, stop asking questions that he can't handle the answers to and tell him to enjoy his hand, since that's the only thing he can be sure of when it comes to where it's been.