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First of all, someone should commend the LW for being honest. A sincere bravo to you. LW, as you know, it's incredibly easy to keep such things a secret in a relationship, especially if there's any inkling that it might adversely affect the significant other. So, bravo. Your past is your past, and while you are not always obligated to reveal your past, your honesty is admirable.
However, it seems your honesty is not being rewarded, and unfortunately, that isn't always the case.
What this all boils down to is thus: there is an aspect about you, LW, that you can't change, an immutable characteristic, if you will. And while your boyfriend accepted and loved everything about you that he knew up until the revealing point of this incident, now, he knows one additional thing about you - and he doesn't love it. And you can't change this characteristic about you. And despite everything he does and used to love about you, he doesn't love this, and he can't get over it. This overshadows everything else that he's come to love about you. And it's not anything you can change. So, essentially, this one thing negates everything else, for him.
Is he being unreasonable? That's not for any of us to decide. Everyone has his or her "dealbreakers." Mine are cheaters. I've been cheated on in the past; I could never date a chronic cheater, even if he's never cheated on me. I would live with the fear forever that every woman that passed through his gaze would be the downfall of our relationship. Irrational? Maybe to you. But it's my call to make, just as much as it is your boyfriend's call to make on the threesome issue.
The problem is, you can't change what you did. Pardon the cliche, but what's done is done. And if he can't accept it, you can't force him to do so. Like I said, everyone's "dealbreaker" is unique and individual to them. If he hasn't made any progress and has further retreated from you for the last six months, I'd say that you'd be better off realizing your relationship has reached its threshold - and the point of no return is way behind you both.