@HopelesslyStupid,
Did he hit you when he found you with the other guy? You kind of glossed over that.
Never mind if you think you deserved it. Violence isn't deserved somehow.
But I will go with it being a poorly-worded post and not violence.
I think you are going to have to come clean with your boyfriend at some point or another, as I think it'll eat you up.
You will need to do this in private, in a non-rushed and unemotional environment and manner. This is not you screaming during a fight, or confessing under duress. This is you calmly saying to him, "We have to talk. I want there to be no secrets between us when we get married. I have one and I want to tell you. I am so sorry I didn't tell you before, but I was afraid I would lose you. I realize that I might lose you now, but I think it's important for you to know the truth."
Then he will undoubtedly ask what it is (and he might get emotional. Here is where you really need to keep calm). "It was months ago, when I had that short fling with __. You asked me if I had told you everything. Well, I hadn't. I gave him oral. We did not sleep together. That is the only thing I have not told you before, and I regret not telling you until now. I did this not to protect him in any way, as he means nothing to me. It was only because I was afraid of losing you. I deeply regret this."
Be prepared for the possibility of him calling this a dealbreaker, by the way, but I think people should know all the facts before they make such a commitment. Plus I feel you will be consumed with guilt if you continue to hold the secret.
I had to really think about this. I know there are people who are going to tell you that no good can come of this, but it is a pretty major issue and, like I said, I think you need to get it out there now and let the chips fall where they may.