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I have a couple of thoughts about this letter. My first thought is that I wholeheartedly agree that you have nothing to apologize over - the past is the past, and, frankly, what's so bad about having a threesome if that's what you wanted? If you regret it for your own reasons, that's fine, but nobody else has a right to regret it for you or to try and convince you that you are "slutty" etc. What, in fact, is "slutty" anyway? Two previous partners? Ten? As long as it's all consensual and you aren't hurting anyone in your present (meaning cheating on somebody), you can sleep with whoever you want. Period.
On the other hand, your boyfriend's reaction is probably not "nuts." As others have suggested, it is more likely insecurity. This is something that can be gotten over, but it doesn't really sound like he's trying. Does he exhibit other possessive behaviors? Is everything REALLY ok except for this one issue? If so, then he might be able to eventually get past it, but if you have other inklings that he doesn't accept you for who you are and who you have been (which are, after all, inextricably related), then I hate to say it, but this guy isn't right for you.
The key thing to remember is that this is HIS problem. He is the one that has to fix this, and if he can't then you have to let him go because above all, you have to protect yourself. You deserve to be with someone who will love you completely - this doesn't mean that he has to accept all of your beliefs about sex, etc, but it does mean that he has to treat you with kindness, and it doesn't sound like this has been happening. It's very sad, but it's not your fault. You didn't do anything wrong here.
And as to all of those people who keep talking about biological urges and whatnot - that's just scientifically incorrect. The question of how men and women are "hard-wired" to act has not yet been settled - scientists can be cited that say that both sexes need to sleep around, that both sexes are naturally monogomous, etc - the fact of the matter is that nobody really knows, and either way it doesn't matter. That's what civilization is - transcending biology. Like it or not, we live in a world where women expect - and deserve - equality, no matter what the biological urge. If men had a biological urge to rape would that be ok? - I think not. So why is biology (especially, I point out again, refuted biology) an excuse for him to take out his jealousy on his girlfriend?
LW, only you know how your bf treats you on a day to day basis. If you think he can get over this, then give him some time. If not, then let him go - you're worth more.