Lifesruff - First, I go against the grain in that I put zero stock in marriage v. dating when we are talking about an agreement between two people to be exclusive. A 3 year committed relationship is longer than some marriages last - the piece of paper changes nothing in the context of staying or going to me or the emotional commitments that you make to each other. A marriage is a legal contract which brings certain legal rights you have that you do not without and a wedding (generally) includes a commitment before a diety - but emotionally a marriage contract changes nothing and whether you stay or go emotionally is premised on precisely the same principles whether the piece of paper exists or not.
That being said, I am 5 years post-D-day and my WS's A started pretty much the same way yours did only his involved a lot of drinking in lieu of MDMA, he knew her a lot better, and that she was married and he knew her spouse as well: they were at a party with a whole bunch of people that I was not at as I was out of town, my WS and his future AP were very drunk and ended up drunkenly talking after everyone else went to bed as my WS had decided to stay there as he was too drunk to drive home, they made out and did all kinds of other stuff, got pretty much naked, and changed their minds at the last second (this is confirmed by text messages I read between them after taking about the "mistake" and how at least they had the presence of mind not to actually have intercourse even though they almost did). He then confessed to me about it the following night and she confessed to her husband, and my WS also spoke to her husband about it, and my WS seemed remorseful, felt terrible, and was generally decent about it and just listened to me when I said nasty things etc, but that didn't last long and ultimately it was rug-swept and nothing else happened between them (or with anyone else I know of) for almost 2 years, when they started their full-blown 2 year+ affair.
I am NOT saying that will happen to you. I am saying that it could, and you need to think about that possibility, and to know that if you want to stay with this person that it is going to take a lot of work on their part, and unfortunately on yours as well. Of course, you could meet someone else and this same thing could happen with them too. You have to decide if this is a history you want to carry with you going forward with him, and to pay attention to not only your boyfriend's behavior but your own. I won't go on an on as you have already received some great advice here - but to the extent you asked if anyone else has been through something similar, I indeed have.
"Sometimes you're going to have to let one person go a thousand different times, a thousand different ways, and there’s nothing pathetic or abnormal about that. You are human." - Heidi Priebe