I am not very good at relationship advice, but if your only problem with making a decision is the fact that she slept with someone when you were apart, then maybe you could try to look at it through your eyes. Think about how the fact that you slept with someone when you were apart, and how it had nothing to do with how you feel about your wife. Then assume that since you sleeping with someone else didn't effect your feelings for your wife, assume that her sleeping with someone didn't effect her feelings for you.
Most likely, she was lonely, and her body needed to be with someone. Since you couldn't be there, she had no choice but to turn to someone else. In all probability, it probably wasn't a very hot, romantic experience for her. In fact, it was more than likely akward for both of them. The first time anyone sleeps together is usally an akward experience *it always was for me at least* They didn't know what each other wanted and needed, and I'm sure that she wasn't in love with this person, or it would've happened more than once.
I know it's hard to get past some things. I quite OFTEN have different standards for myself than for my husband, because I know what I'm thinking, but I tend to assume the worst of everyone else.
I'm sure you don't really want to know the details, and I'm sure she doesn't want to share them with you, but do your best to imagine them not having a wonderful experience together. Maybe if you quit imagining a glamorous view of them together, and look at the reality that it probably was a big fat way for her to releive some tension, you could probably forget all about it.
It sounds like you think she is worth "getting over it" for
Then again, I'm not good with relationship advice.