Part of me wants to say the usual....
"Dear Penthouse Letters"
But I can see where this could happen.
Big Rock, one thing about swinging for a while is that sex isn't casual sex, its casual period. Casual sex means no strings attached, while casual in this case would be like watching the football game. Its still fun, intense, etc, but its more like a game.
You become desensitized to sex, its simply not a big deal to you, and it can be completely detached from love and jealousy. Its one of the better parts about swinging in that you don't have those insecurities and you know where your relationship is not simply based on sex.
The problem though is that while shes desensitized to it, she doesn't think you are. So she meets this guy she really likes, then loves, etc, but shes terrified if she tells him about what she does he will thinks shes a slut, and leave. This is not an unreasonable fear, because if the man is still in jealousy mode, thats exactly what he will think.
So she doesn't tell you about her past, afraid of your reaction to it, yet she doesn't think its a big deal either, and she likes her friends shes made.
So she has one last 'fling' almost equivalent to a bacheloret party, doing what they casually do, which is have sex, not worried about your feelings not out of callousness but almost out of a sense of superiority. SHE knows its not a big deal, and she still loves you, and its not like she hasn't done it 100 times already. Maybe she thinks in time she will bring you to her way of thinking.
Now this is not to excuse her behavior, its to explain it. What she did was wrong even to us swingers, and if I were one of her friends at her 'last fling' I'd have been uncomfortable knowing you didn't know about it. Even though my wife and I have been swingers, I'd be rather upset if I came home to see her getting tagged teamed and she never said anything to me about it.
As to what to do, I think you have two choices.
If this lifestyle really interests you, and you can look past her behavior, it might be worth talking to her about getting back together.
If it doesn't, I'd most likely bail. The reason being that obviously she doesn't want to leave the lifestyle, and even if she says she will, I think there will be long term conflict, not to mention thats a lot of stress on a young marriage.